DBMS Disadvantages

8/10/2019 DBMS Disadvantages

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The DBMS may also have some minor disadvantages:

1. Increased costs

Database systems require sophisticated hardware and software and highly skilled personnel. The

cost of maintaining the hardware, software, and personnel required to operate and manage a database

system can be substantial. Training, licensing, and regulation compliance costs are often overlookedwhen database systems are implemented.

Cost of Hardware & Software

A processor with high speed of data processing and memory of large size is required to run the

DBMS software. It means that you have to upgrade the hardware used for file-based system. Similarly,

DBMS software is also very costly.

Cost of Data ConversionWhen a computer file-based system is replaced with adatabase system, the data stored into data file

must be converted to database file. It is very difficult and costly method to convert data of data files into

database. You have to hire database and system designers along with application programmers.

Alternatively, you have to take the services of some software house. So a lot of money has to be paid for

developing software.

Cost of Staff Trailing (Appointing Technical Staff)

Most DBMSs are often complex systems so the training for users to use the DBMS is required.

Training is required at all levels, including programming, application development, and databaseadministration. The organization has to be paid a lot of amount for the training of staff to run the DBMS.

The trained technical persons such as database administrator, application programmers, data

entry operators etc. are required to handle the DBMS. You have to pay handsome salaries to these

persons. Therefore, the" system cost increases.

2. Database Damage

In most of the organizations, all data is integrated into a single database. If database is damaged due to

electric failure or database is corrupted on the storage media, then your valuable data may be lost

forever or whole system stops.

3. Management complexity

Database systems interface with many different technologies and have a significant impact on a

companys resources and culture. The changes introduced by the adoption of a database system must

be properly managed to ensure that they help advance the companys objectives. Given the fact that
http://sql-databases.blurtit.com/q315992.htmlhttp://sql-databases.blurtit.com/q315992.html


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Record: The related fields of data are grouped to form a record. Thus, a record represents a

collection of attributes that describe an entity. Fixed-length records contain, a fixed number of

fixed-length data fields. Variable-length records contain a variable number of fields and field

lengths.

File:A group of related records is known as a data file, or table. Files are frequently classified by

the application for which they ar primarily used, such as a payroll file or an inventory file, or the

type of data they contain, such as a document file or a graphical image file. Files are also

classified by their permanence, for example, a master file versus a transaction file. A transaction

file would contain records ofall transactions occurring during a period, whereas a master file

contains all the permanent records. A history file is an obsolete transaction or master file

retained for backup purposes or for long-term historical storage called archival storage.

Database:It is an integrated collection of logically related records or objects. A

database consolidates records previously stored in separate files into a common pool of data

records that provides data for many applications. The data stored in a database is independent

of the application programs using it and o the type ofsecondary storage devices on which it is

stored.

Q. 2. What are the various characteristics of DBMS?

Ans. The major characteristics of database approach are:

Self-describing Nature of a Database System

Insulation between Programs and Data, and DataAbstraction

Support of Multiple Views of the Data

Sharing of Data and Multi user Transaction Processing

Q. 3. What are the various characteristics of DBMS approach?

Ans.

1. Self-contained nature

DBMS system contains data plus a full description of the data (called metadata) metadata is data

about data - data formats, record structures, locations, how to access, indexes metadata is stored in a


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catalog and is used by DBMS software to know how to access the data. Contrast this with the file

processing approach where application programs need to know the structure and format of records and

data.

2. Program-data independence

Data independence is immunity of application programs to changes in storage structures and access

techniques. E.g. adding a new field, changing index structure, changing data format, In a DBMS

environment these changes are reflected in the catalog. Applications arent affected. Traditional file

processing programs would all have to

change, possibly substantially.

3. Data abstraction

A DBMS provides users with a conceptual representation of data (for example, as objects with

properties and inter-relationships). Storage details are hidden. Conceptual representation is provided in

terms of a data model.

4. Support for multiple views

DBMS may allow different users to see different views of the DB, according to the perspective each

one requires. E.g. a subset of the data - For example; the people using the payroll system need

not/should not see data about students and class schedules. E.g. data presented in a different form from

the way it is stored - For example someone interested in student transcripts might get a view which is

formed by combining information from separate files or tables.


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5. Centralized control of the data resource

The DBMS provides centralized control of data in an organization.

This brings a number of advantages:

(a) reduces redundancy

(b) avoids inconsistencies

(c) data can be shared

(d) standards can be enforced

(e) security restrictions can be applied

(f) integrity can be maintained

a, b. Redundancy and Inconsistencies

Redundancy is unnecessary duplication of data. For example if accounts department and registration

department both keep student name, number and address.

Redundancy wastes space and duplicates effort in maintaining the data.

Redundancy also leads to inconsistency.

Inconsistent data is data which contradicts itself - e.g. two different addresses for a given student

number. Inconsistency cannot occur if data is represented by a single entry (i.e. if there is no

redundancy).

Controlled redundancy: Some redundancy may be desirable (for efficiency). A DBMS should be aware

of it, and take care of propagating updates to all copies of a data item.

This is an objective, not yet currently supported.

c. Sharing


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Need concurrency control

Multiple user views

d. Standards

E.g. data formats, record structures, naming, documentation

International, organizational, departmental ... standards

e. Security

- restricting unauthorized access

DBMS should perform security checks on all accesses.

f. Integrity

Maintaining validity of data;

e.g. employee numbers must be in some range

e.g. every course must have an instructor

e.g.. student number must be unique

e.g. hours worked cannot be more than 150

These things are expressed as constraints.

DBMS should perform integrity checks on all updates. Currently DBMSs provide limited integrity

checks.

Q. 3. What are the various types of databases?

Ans. Types of Databases

Continuing developments in information technology and its business applications have resulted in

the evolution of several major types of databases. Several major conceptual categories of databases that

may be found in computer-using organizations include:


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Operational Databases

The databases store detailed data needed to support the operations of the entire organization. They

are also called subject area databases (SADB), transaction databases, and production databases:

Examples are customer databases, personnel databases, inventory databases, and other databases

containing data generated by business operations

Distributed Databases

Many organizations replicate and distribute copies or parts of databases to network

sewers at a variety of sites. These distributed databases can reside on network servers

on the World Wide Web, on corporate Intranets or extranets, or on other company networks.

Distributed databases may be copies of operational or analytical. databases,

hypermedia or discussion databases, or any other type of database. Replication and distribution of

databases is done to improve database performance and security.

External Databases

Access to external, privately owned online databases or data banks is available for a fee to end users

and organizations from commercial online services, and with or without charge from many sources on

the Internet, especially the Web.

Hypermedia Databases

It consists of hyperlinked pages of multimedia (text, graphics, and photographic images, video clips,

audio segments, etc.). From a database management point of view, the set of interconnectedmultimedia pages at a website is a database of interrelated hypermedia page elements, rather than

interrelated data records.

Q. 4. What do you mean by DBMS?

Ans. A DBMS is best described as a collection of programs that manage the database structure and

that control shared access to the data in the database. Current DBMSes also store the relationships

between the database components; they also take care of defining the required access paths to those

components

A database management system (DBMS) is the combination of data, hardware, software and users to

help an enterprise manage its operational data.


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The main function of a DBMS is to provide efficient and reliable methods of data retrieval to many

users. Efficient data retrieval is an essential function of database systems. DBMS must be able to deal

with several users who try to simultaneously access several items and most frequently, the same data

item A DBMS is a set of programs that is used to store and manipulation data that include the following:

Adding new data, for example adding details of new student.

Deleting unwanted data, for exampledeleting the details of students who have

completed course.

Changing existing data, for example modifying the fee paid by the student.

A database is the information to be stored whereas the database management system is the system

used to manage the database. . This structure may be regarded in terms of its hardware

implementation, called the physical structure, or this structure may be regarded independently of its

hardware implementation, called the logical structure. In either case, the data structure is regarded as

static because a database cannot process anything. The DBMS is regarded as dynamic because it is

through the DBMS that all database processing takes place. How the DBMS presents data to the user is

called the view structure.

There are two general modes for data use: queries and transactions. Both forms use the DBMS for

processing. The query is processed for presentation in views and none of these processes are written to

the database. The transactional is processed for updating values in the database variables. These

updates are written to the database. A DBMS provides various functions like data security, data

integrity, data sharing, data concurrence, data independence, data recovery etc. However, all database

management systems that are now available in the market like Sybase, Oracle, and MS-Access do not

provide the same set of functions, though all are meant for data management.

Q. 5. What are the various components of DBMS?

Ans.Basic Components: A database system has four components. These four

components are important for understanding and designing the database system. These

are:

1. Data

2. Hardware


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3. Software

4. Users

1. Data

As we have discussed above, data is raw hand information collected by us. Data is made up of data

item or data aggregate. A Data item is the smallest unit of named data: It may consist of bits or bytes. A

Data item is often referred to as field or data element. A Data aggregate is the collection of data items

within the record, which is given a name and referred as a whole. Data can be collected orally or

written. A database can be integrated and shared. Data stored in a system is partition into one or two

databases. So if by chance data lost or damaged at one place, then it can be accessed from the second

place by using the sharing facility of data base system. So a shared data also cane be reused according to

the users requirement. Also data must bein the integrated form. Integration means data should be in

unique form i.e. data collected by using a well-defined manner with no redundancy, for example Roll

number in a class is non-redundant form and so these have unique resistance, but names in class may

be in the redundant form and can create lot of problems later on in using and accessing the data.

2. Hardware

Hardware is also a major and primary part of the database. Without hardware nothing can be done.

The definition of Hardware is which we can touch and see, i.e. it has physical existences. All physical

quantity or items are in this category. For example, all the hardware input/output and storage devices

like keyboard, mouse, scanner, monitor, storage devices (hard disk, floppy disk, magnetic disk, and

magnetic drum) etc. are commonly used with a computer system.

3. Software

Software is another major part of the database system. It is the other side of hardware. Hardwareand software are two sides of a coin. They go side by side. Software is a system. Software are further

subdivided into two categories, First type is system software (like all the operating systems, all the

languages and system packages etc.) and second one is an application software (payroll, electricity

billing, hospital management and hostel administration etc.). We can define software as which we

cannot touch and see. Software only can execute. By using software, data can be manipulated,

organized and stored. -

4. Users

Without user all of the above said components (data, hardware & software) are meaning less. User

can collect the data, operate and handle the hardware. Also operator feeds the data and arranges thedata in order by executing the software. Other components

1. People - Database administrator; system developer; end user.

2. CASE tools: Computer-aided Software Engineering (CASE) tools.

3. User interface - Microsoft Access; PowerBuilder.


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4. Application Programs - PowerBuilder script language; Visual Basic; C++; COBOL.

5. Repository - Store definitions of data called METADATA, screen and report formats, menu

definitions, etc.

6. Database - Store actual occurrences data.

7. DBMS - Provide tools to manage all of this - create data, maintain data, control security access to

data and to the repository, etc.

Q. 6.What are the various functions of DBMS?

Ans.These functions will include support for at least all of the following:

Data definition:The DBMS must be able to accept data definitions (external schemas, the

conceptual schema, the internal schema, and all associated mappings) in source form and convert them

to the appropriate object form.

Data manipu1ation:The DBMS must be able to handle requests from the users to retrieve, update,

or delete existing data the database, or to add new data to the database. In other words, the DBMS

must include a data manipulation language (DML) processor component.

Data security and integrity:The DBMS must monitor user requests and reject

any attempt to violate the security and integrity rules defined by the DBA.

Data recovery and concurrency:The DBMS - or else some other related software component,

usually called the transaction manager - must enforce certain recovery and concurrency controls.

Data Dictionary: The DBMS must provide a data dictionary function. The data dictionary can be

regarded as a database in its own right (but a system database, rather than a user database). The

dictionary contains data about the data (sometimes called metadata) - that is, definitions of other

objects in the system - rather than justraw data. In particular, all the various schemas and mapping

(external, conceptual, etc.) will physically be stored, in both source and object form, in the dictionary. Acomprehensive dictionary will also include cross- reference information, showing, for instance, which

programs use which pieces of the database, which users require which reports, which terminals are

connected to the system, and so on. The dictionary might even - in fact, probably shouldbe

integrated into the database it defines, and thus include its own definition. It should certainly be

possible to query the dictionary just like any other database, so that, for example, it is possible to tell

which programs and or users are likely to be affected by some proposed change to the system.


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Performance: It goes without saying that the DBMS should perform all of the functions identified

above as efficiently as possible.

Q7. What are the advantages and disadvantages of a database approach?

Ans.ADVANTAGES OF DBMS

One of the major advantages of using a database system is that the organization

can be handled easily and have centralized management and control over the data by

the DBA. Some more and main advantages of database management system are given

below:

The main advantages of DBMS are:

1. Controlling Redundancy

In a DBMS there is no redundancy (duplicate data). If any type of duplicate data arises, then DBA can

control and arrange data in non-redundant way. It stores the data on the basis of a primary key, which is

always unique key and have non-redundant information. For example, Roll no is the primary key to store

the student data.

In traditional file processing, every user group maintains its own files. Each group independently keeps

files on their db e.g., students. Therefore, much of the data is stored twice or more. Redundancy leads

to several problems:

Duplication of effort

Storage space wasted when the same data is stored repeatedly

Files that represent the same data may become inconsistent (since the updates are applied

independently by each users group).We can use controlled redundancy.

2. Restricting Unauthorized Access

A DBMS should provide a security and authorization subsystem.


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Some db users will not be authorized to access all information in the db (e.g., financial data).

Some users are allowed only to retrieve data.

Some users are allowed both to retrieve and to update database.

3. Providing Persistent Storage for Program Objects and Data Structures

Data structure provided by DBMS must be compatible with the programming languages data

structures. E.g., object oriented DBMS are compatible with programming languages such as C++, SMALL

TALK, and the DBMS software automatically performs conversions between programming data structure

and file formats.

4. Permitting Inferencing and Actions Using Deduction Rules

Deductive database systems provide capabilities for defining deduction rules for inferencing new

information from the stored database facts.

5. Inconsistency can be reduced

In a database system to some extent data is stored in, inconsistent way. Inconsistency is another

form of delicacy. Suppose that an em1oyee Japneet work in department Computer is represented

by two distinct entries in a database. So way inconsistent data is stored and DBA can remove this

inconsistent data by using DBMS.

6. Data can be shared

In a database system data can be easily shared by different users. For example, student data can be

share by teacher department, administrative block, accounts branch arid laboratory etc.

7. Standard can be enforced or maintained

By using database system, standard can be maintained in an organization. DBA is overall controller of

database system. Database is manually computed, but when DBA uses a DBMS and enter the data in

computer, then standard can be enforced or maintained by using the computerized system.

8. Security can be maintained

Passwords can be applied in a database system or file can be secured by DBA. Also in a database

system, there are different coding techniques to code the data i.e. safe the data from unauthorizedaccess. Also it provides login facility to use for securing and saving the data either by accidental threat or

by intentional threat. Same recovery procedure can be also maintained to access the data by using the

DBMS facility.

9. Integrity can be maintained


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In a database system, data can be written or stored in integrated way. Integration means unification

and sequencing of data. In other words it can be defined as the data contained in the data base is both

accurate and consistent. Data can be accessed if it is

compiled in a unique form. We can take primary key ad some secondary key for integration of data.

Centralized control can also ensure that adequate checks are

incorporated in the DBMS to provide data integrity.

10. Confliction can be removed

In a database system, data can be written or arranged in a well-defined manner by DBA. So there is

no confliction between the databases. DBA select the best file structure and accessing strategy to get

better performance for the representation and use of the

data.

11. Providing Multiple User Interfaces

For example query languages, programming languages interfaces, forms, menu- driven interfaces,

etc.

12. Representing Complex Relationships Among Data

It is used to represent Complex Relationships Among Data

13. Providing Backup and Recovery

The DBMS also provides back up and recovery features.

DISADVANTAGES OF DBMS

Database management system has many advantages, but due to some major problem

arise in using the DBMS, it has some disadvantages. These are explained as:

1.Cost

A significant disadvantage of DBMS is cost. In addition to the cost of purchasing or developing the

software, the organization *111 also purchase or upgrade the hardware

and so it becomes a costly system. Also additional cost occurs due to migration of data

from one environment of DBMS to another environment.


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2. Problems associated with centralization

Centralization also means that data is accessible from a single source. As we know the centralized

data can be accessed by each user, so there is no security of data from unauthorized access and data

can be damaged or lost.

3. Complexity of backup and recovery

Backup and recovery are fairly complex in DBMS environment. As in a DBMS, if you take a backup of

the data then it may affect the multi-user database system which is in operation. Damage database can

be recovered from the backup floppy, but iterate duplicity in loading to the concurrent multi-user

database system.

4. Confidentiality, Privacy and Security

When information is centralized and is made available to users from remote locations, the

possibilities of abuse are often more than in a conventional system. To reduce the chances of

unauthorized users accessing sensitive information, it is necessary to take technical, administrative and,

possibly, legal measures. Most, databases store valuable information that must be protected against

deliberate trespass and destruction.

5. Data Quality

Since the database is accessible to users remotely, adequate controls are needed to control users

updating data and to control data quality. With increased number of users accessing data directly, there

are enormous opportunities for users to damage the data. Unless there are suitable controls, the data

quality may be compromised.

6. Data Integrity

Since a large number of users could be using .a database concurrently, technical safeguards are

necessary to ensure that the data remain correct during operation. The main threat to data integrity

comes from several different users attempting to update the same data at the same time. The database

therefore needs to be protected against inadvertent changes by the users.

7. Enterprise Vulnerability

Centralizing all data of an enterprise in one database may mean that the database becomes an

indispensable resource. The survival of the enterprise may depend on reliable information being

available from its database. The enterprise therefore becomes vulnerable to the destruction of thedatabase or to unauthorized modification of the database.

8. The Cost of using a DBMS

Conventional data processing systems are typically designed to run a number of well-defined,

preplanned processes. Such systems are often tuned to run efficiently for the processes that they

were designed for. Although the conventional systems are usually fairly inflexible in that new


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At end we can sat that a DBMS is a piece of software that is designed to make the processing faster

and easier.

Q 9 Describe major advantages of a database system over file system Or Discuss the DBMS and File

processing system Also give the limitations of file processing system

Ans.TRADITIONAL FILE PROCESSING

Data are organized, stored, and processed in independent files of data records. In the traditional file

processing approach, each business application was designed to use one or more specialized data files

containing only specific types of data records

TRADITIONAL FILE SYSTEM OR FILE ORIENTED APPROACH

The business computers of 1980 were used in processing of business records and produce

information using file oriented approach or file processing environment At that time that system was

reliable and faster than the manual system of record keeping and processing In this system the data is

organized in the form of different files. Since that system was the collection of files - so we can say it was

a file-oriented system. Following terms was commonly used in this approach or the features of File

oriented system.

1. Master file

The file that is created only once i.e. at the starting of computerization or a file which rarely changes.

For example: In a bank master file the account no, name and balance are entered only once and less

frequently changes.

2. File activity ratio

The number of records processed one run divided by total number of records. For example: if we

changes 100 records from a bank file containing 200 records then file activity ratio is 100/200 0.5. It

should be noted that this ratio of master file is less.

3. Transaction file

A file that is created repeatedly after regular interval of time. For example: the payroll file of

employee is updated at the end of every month.

4. File volatility ratio


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It is the number of records updated in a transaction file divided by total number of records. The file

volatility ratio of transaction file is very high.

5. Work file

A temporary file that helps in sorting and merging of records from one file to other.

6. File organization

It means the arrangement of records in a particular order. There were three types of file

organizations

1.

Sequential

2.

Direct

3.

Indexed sequential

7. Data island

In this system each dept has its own files designed for local applications. Each department has its

own data processing staff, set of policies, working rules and report formats. It means programs were

depending on the file structure or format of file. If the structure of file changes, the program has also to

be changed. These days the file oriented approach is still used but has following limitations:

LIMITATIONS OF FILE ORIENTED APPROACH

Duplicate data

Since all the files are independent of each other. So some of the fields or files are stored more than

once. Hence duplicacy is more in case of file approach but dbms has controlled duplicacy.

Separated and isolated data

To make a decision, a user might need data from two separate files. First, analysts and

programmers to determine the specific data required from each file and the relationships between the

data evaluated the files. Then applications could be written in a third generation language to process

and extract the needed data. Imagine the work involved if data from several files was needed!

Inconsistency

In this system, data is not consistent. If a data item is changed the all the files containing that dataitem need to be changed and updated properly. If all the files are not updated properly there may be

high risk of inconsistency. DBMS have data consistency.

Poor data integrity


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A collection of data has integrity. A file is said to be have data integrity - it means a item is not be

stored in duplicate manner. It has been seen that file oriented system have poor data integrity control.

Data integrity has been achieved in DBMS.

Every operation is programmable

The processing tasks like searching, editing, deletion etc should have separate programs. It means

there were no functions available for these operations. DBMS have ready-made commands for such

operations.

Data inflexibility

Program-data interdependency and data isolation limited the flexibility of file processing systems in

providing users with ad hoc information requests. Because designing applications was so programming-

intensive, MIS department staff usually restricted information requests Therefore, users often resorted

to manual methods to obtain needed information.

Concurrency problem

It means using a same record at same time. This problem was common in file approach but can be

controlled in DBMS.

Application programs are dependent on the file format:

In file processing system the physical formats of the files are entered in the programs. The change in

file means change in program and vice versa. No such problem in DBMS.

Poor data security

All the files are stored in the flat form or text files. These files can be easily located and trapped

because file approach, has no data security.

Difficult to represent the complex objects:

Some the objects may be of variable length records can be computerized using this approach. DBMS

has capability to handle fixed-length records as well as variable-length records.

Can not support heavy databases:

The databases on the Internet can be handled by the files system - but DBMS like oracle is used forheavy data base applications. On the other hand the DBMS have following advantages.

Difficulty in representing data from the users view

To create useful applications for the user, often data from various files must be combined. In file

processing it was difficult to determine relationships between isolated data in order to meet user

requirements.


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PROBLEMS OF FILE PROCESSING

The file processing approach finally became too cumbersome, costly, and inflexible to supply the

information needed to manage modem businesses. It was replaced by the database management

approach. File processing systems had the following major problems:

Data Redundancy

Independent data files included a lot of duplicated data; the same data was recorded

and stored in several files. This data redundancy caused problems when data had to

be updated, since separate file maintenance programs had to be developed and

coordinated to ensure that each file was properly updated. Unfortunately, a lot of

inconsistencies occurred among data stored in separate files.

Lack of Data Integration

Having independent files made it difficult to provide end users with information for

ad hoc requests that required accessing data stored in several different files. Special

computer programs had to be written to retrieve data from each independent file. This

was so difficult, time-consuming, and costly for some organizations that it was

impossible to provide end users or management with such information.

Data Dependence

In file processing systems, major components of the system - the organization of files,

their physical locations of storage hardware, and the application software used to

access those files depended on one another in significant ways. Changes in the

format and structure of data and records in a file required that changes be made to all

of the programs that used that file. This program maintenance effort was a major


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burden of file processing systems.

Other Problems

It was easy for data elements to be defined differently by different end users and

applications. Integrity of the data was suspect because there was no control over their

use and maintenance by authorized end users.

Q.10. What are the various types of database uses?

Ans. Without user all o the above said components (data, hardware & software) are meaning less.

User can collect the data, operate and handle the hardware. Also operator feeds the data and arranges

the data in order by executing the software. Users are of mainly of four types. These are:

(a) Nave user

Nave user has no knowledge of database system and its any supporting software. These are used at

the end form. These are like a layman, which have little bit knowledge or computer system. These users

are mainly used for collecting the data on the notebooks or on the pre-deigned forms. An automated

teller machine (ATMs) user are in these categories. Nave user can work on any simple GUI base menu

driven system. Internet using non-computer based person are in this form.

(b) End User or Data Entry Operators

Data entry operators are preliminary computer based users. The function of data entry operators are

only to operate the computer (start! stop the computer) and feed or type the collected information

(data) in menu driven application program and to execute it according to the analyst requirement.

These user are also called On line users. These user communicate the database directly via an on line

terminal or indirectly via a user interface. These users require certain amount of expertise in the

computer programming language, but require complete knowledge of computer operations.

(c) Application programmer

He is also called simple programmer. The working of application programmer is to develop a new

project i.e. program for a particular application or modify an existing program. Application programmer

works according to some instructions given by database administrator (DBA). Application programmer

can handle all the programming language like Fortran, Cobol, dbase etc.

(d) DBA (Data Base Administrator)


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DBA is a major user. DBA either a single person or a group of persons. DBA is only the custodian of

the business firm or organization but not the owner of the organization. As bank manager is the DBA of

a bank, who takes care about the bank money and not use it. Only DBA can handle the information

collected by end user and give the instructions to the application programmer for developing a new

program or modifying an existing program. DBA is also called an overall controller of the organization. In

computer department of a firm either system analysts or an EDP (Electronic Data Processing) Manager

works as DBA. In other words DBA is the overall controller of complete hardware and software.

RESPONSIBILITIES OF DBA

As we know DBA is the overall commander of a computer system, so it has number of duties, but

some of his/her major responsibilities are as follows:

1.

DBA can control the data, hardware, and software and gives the instructions to the application

programmer, end user and naive user.

2.

DBA decides the information contents of the database. He decides the suitable database file

structure for arrangement of data. He/She uses the proper DDL techniques.

3.

DBA compiles the whole data in a particular order and sequence.4.

DBA decides where data can be stored i.e. take decision about the storage structure.

5.

DBA decides which access strategy and technique should be used for accessing the data.

6.

DBA communicates with the user by appropriate meeting, DBA co-operates with

user.

1.

DBA also define and, apply authorized checks and validation procedures.

2.

DBA also takes backup of the data on a backup storage device so that if data can be lost then it

can be again recovered and compiled. DBA also recovers the damaged data.

3.

DBA also changes the environment according to user or industry requirement and monitor the

performance.4.

DBA should be good decision-maker. The decision taken by DBA should be correct, accurate &

efficient.

5.

DBA should have leadership quality.

6.

DBA liaise with the user in the business to take confidence of the customer about availability of

data.

Q11. Discuss the architecture of database management system.

Ans. DBMS ARCHITECTURE


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There are many different framework have been suggested for the DBMS over the last several year.

The generalized architecture of a database system is called ANSI/SPARC (American National Standards

Institute/Standards Planning and Requirements Committee) model.

In 1972, a final report about database is submitted by ANSI (American National Standard Institute)

and SPARC (Standard Planning And Requirement Committee). According to this approach, three levels of

a database system was suggested and they are:

External view (Individual user view)

Conceptual View (Global or community user view)

Internal level (physical or storage view).

For the system to be usable, it must retrieve data efficiently. This concern has led to the design of

complex data structures for the representation of data in the database. Since many database systems

users are not computer trained, developers hide the complexity from users through several levels of

abstraction, to simplify users interactions with the system.

These three views or levels of the architecture are as shown in the diagram as follows:

OBJECTIVES OF THREE LEVEL ARCHITECTURE

The database views were suggested because of following reasons or objectives of levels of a

database:


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1. Make the changes easy in database when some changes needed by environment.

2. The external view or user views do not depend upon any change made ii other view. For example

changes in hardware, operating system or internal view should not change the external view.

3. The users of database should not worry about the physical implementation and internal working

of database system.

4. The data should reside at same place and all the users can access it as per their requirements.

5. DBA can change the internal structure without effecting the users view.

6. The database should be simple and changes can be easily made.

7. It is independent of all hardware and software.

All the three levels are shown below

External/View level:

The highest level of abstraction where only those parts of the entire database are included which are

of concern to a user. Despite the use of simpler structures at the logical level, some complexity remains,

because of the large size of the database. Many users of the database system will not be concerned with

all this information. Instead, such users need to access only a part of the database. So that theirinteraction with the system is simplified, the view level of abstraction is defined. The system may

provide many views for the same database.

Databases change over time as information is inserted and deleted. The collection of information

stored in the database at a particular moment is called an instance of the database. The overall design

of the database is called the database schema. Schemas are changed infrequently, if at all.


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Database systems have several schemas, partitioned according to the levels of abstraction that we

discussed. At the lowest level is the physical schema; at the intermediate level is the logical schema and

at the highest level is a subschema.

The features of this view are

The external or user view is at the highest level of database architecture.

Here only one portion of database will be given to user.

One portion may have many views.

Many users and program can use the interested part of data base.

By creating separate view of database, we can maintain security.

Only limited access (read only, write only etc) can be provided in this view.

For example: The head of account department is interested only in accounts but in library

information, the library department is only interested in books, staff and students etc. But all such data

like student, books, accounts, staff etc is present at one place and every department can use it as per

need.

Conceptual/Logical level

Database administrators, who must decide what information is to be kept in the database, use this

level of abstraction. One conceptual view represents the entire database. There is only one conceptual

view per database.

The description of data at this level is in a format independent of its physical representation. It also

includes features that specify the checks to retain data consistence and integrity.

The features are:

The conceptual or logical view describes the structure of many users.

Only DBA can be defined it.

It is the global view seen by many users.

It is represented at middle level out of three level architecture.

It is defined by defining the name, types, length of each data item. The create table

commands of Oracle creates this view.

It is independent of all hardware and software.


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Internal/Physical level

The lowest level of abstraction describes how the data are stored in the database, and what

relationships exist among those data. The entire database is thus described in terms of a small number

of relatively simple structures, although implementation of the simple structures at the logical level may

involve complex physical-level structures, the user of the logical level does not need to be aware of this

complexity.

The features are :

It describes the actual or physical storage of data.

It stores the data on hardware so that can be stored in optimal time and accessed

in optimal time.

It is the third level in three level architecture.

It stores the concepts like:

B-tree and Hashing techniques for storage of data.

Primary keys, secondarykeys, pointers, sequences for data search.

Data compression techniques.

It is represented as

FILE EMP [

INDEX ON EMPNO

FIELD = {

(EMPNO: BYTE (4),

ENAME BYTE(25))]

Mapping between views

The conceptual/internal mapping:

defines conceptual and internal view correspondence

specifies mapping from conceptual records to their stored counterparts

An external/conceptual mapping:


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defines a particular external and conceptual view correspondence

A change to the storage structure definition means that the conceptual/internal

mapping must be changed accordingly, so that the conceptual schema may remain

invariant, achieving physical data independence.

A change to the conceptual definition means that the conceptual/external mapping

must be changed accordingly, so that the external schema may remain invariant,

achieving logical data independence.

Q. 12. Write a note on Database Language And Interfaces.

Ans.Some main types of languages and facilities are provided by DBMS.

1. Programming Language

2. Data Manipulation Language

3. Data Definition Language

4. Schema Description Language

5. Sub-Schema Description Language

6. SQL (Structured Query Language)

1. Programming Language

All the programming language like Cobol, Fortran, C, C++, Pascal etc. has syntax and semantics. These

all have structured and logical structure, so these all commonly used to solve general and scientificproblems. All the business-oriented problems can be solved by the three GL and Fourth Gt.

2. DML

Some language that gives instructions to the programming language and other languages is called

data manipulation language (DML). This language creates interface (linkage) between user and

application program. This is extension of the program of the language used to manipulate data in the


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database. DML involves retrieval of data from the database, insertion of new data into the database

and deletion or modification of the existing data. Some data manipulation operations are also called

QUERY or QUERY OPERATIONS. A Query is a statement in DML that request the retrieval of data from

the database i.e. to search the data according to the user requirement. The subset of the DML used to

operate the query is known as Query Language. DML provides commands to select & retrieve data from

the database. Commands used in the DML are to insert, to update & to delete the records. The

commands have different syntax for different programming language. For example, Fortran, Cobol, C

etc. provide such type of facility with the help of database management system. The data manipulation

function provided by DBMS can be invoked in a application program directly by procedural calls or by

processors statement. This procedure can be done by the compiler. The DML can become

procedural language according to the user requirement. If the DML is non-procedural than user will

indicate only what is to be retrieved. In both the cases the DBMS optimize the exact answer by using

DML.

3. DDL

Database management system provides a facility known as Data Definition Language or datadescription language (DDL). DDL can be used to define conceptual schema (Global) and also give some

details about how to implement this schema in the physical devices used to store the data. The

definition includes all the entity sets and their associated attributes as well as the relationship among

the entities set. The definitions also have some constraints which are used in DML. DDL also have some

meta-data (it is data about the data in database). Meta-data have data dictionary, directory, system

catalog to describe data about data. The dictionary contains the information about the data stored in

the database and it is consulted by DBMS before any data manipulation operations. The DBMS maintain

the information on the file structure and also used some access method to access the data efficiently.

DDL is used for the help of DML.

We can say that there is another language - Data Sub Language (DSL) which is the

combination of both DML and DDL.

DSL = DML + DDL

4. Schema Description Language (SDL) or Schema

It is necessary to describe the organization of the data in a formal manner. The logical and physical

database descriptions are used by DBMS software. The complete and overall description of data is

referred to as schema. The schema and subschema words are brought into DBMS by CODASYL

(Conference on data system language committee) and also by the CODASYLs database task group.Schema is also referred to as conceptual model or global view (community view) of data. Suppose a

complete description of collected data having all classes and student data, all employees (teaching &

non-teaching) data and other concept of data related to the college is called Schema of the college. We

can say that we relate whole college data logically, which is called schema.


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5 Sub Schema Description language

The term schema is used to mean an overall chart of the data items, types and record type stored in

a database. The term sub-schema refers to an application programmers view of data he uses. Sub-

schema is the part of schema. Many different sub-schemas can be derived from one schema. An

application programmer does not use whole data i.e. full schema, e.g. As in an organization, purchase-

order for the maintenance department is the sub-schema of the whole schema description of

the purchase department in the hole industry. Two or more than two application- programmers use the

different sub-schemas. One person named A uses the sub-schema purchase-order whereas programmer

B uses the sub-schema supplier. Their operations and views are different according to their own sub-

schema but both combined these two sub-schemas on the basis of a common key.

6. Structured Query Language (SQL):

SQL organized with the system R. System R means it is relational language. SQL is also called

Structure Query Language. This language was developed in 1974 at IBMs San Jose Research Center. The

purpose of this language is to provide such non-procedural commands which are used for validation of

the data and for searching the data. By using this language we can do any query about the data. SQL is

sometimes named by SQUARE language. This language was helpful for both DDL and DML for the system

R. Some SQL are also called Relational languages and used in a commercial RDBMS. Some commonly

used SQL are ORACLE, INGRES, SYBASE etc. SQL resembles relational algebra and relational calculus in a

relational system approach.

DBMS INTERFACES

Types of interfaces provided by the DBMS include:

Menu-Based interfaces for Web Clients or Browsing

Present users with list of options (menus)

Lead user through formulation of request

Query is composed of selection options from menu displayed by system.

Forms-Based Interfaces

Displays a form to each user

User can fill out form to insert new data or fill out only certain entries.

Designed and programmed for nave users as interfaces to canned transactions.


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Graphical User Interfaces

Displays a schema to the user in diagram form. The user can specify a query by manipulating the

diagram. GUIs use both forms and menus.

Natural Language Interfaces

Accept requests in written English or other languages and attempt to understand them.

Interface has its own schema, and a dictionary of important words. Uses the schema and dictionary

to interpret a natural language request.

Interfaces for Parametric Users

Parametric users have small set of operations they perform.

Analysts and programmers design and implement a special interface for each class of nave users.

Often a small set of commands included to minimize the number of keystrokes required. (I.e.

function keys)

Interfaces for the DBA

Systems contain privileged commands only for DBA staff.

Include commands for creating accounts, setting parameters, authorizing accounts,

changing the schema, reorganizing the storage structures etc.

Q.13. Describe the Classification of Database Management Systems.

Ans.Categories of DBMS

DBMS (Database Management System)

It is software to manage many databases. A DBMS is a software component or logical tool to handle

the databases. All the queries from user about the data stored in the database will be handled by DBMS.

There are many DBMSs available in market like dBase, FoxBASE, FoxPro, Oracle, Unify, Access etc.

RDBMS (Relational Data Base Management System)


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Each database system uses a approach to store and maintain the data. For this purpose three data

models were developed like Hierarchical model, Network Model and Relational Model. In the

hierarchical model the data were arranged in the form of trees, in network model the data was arranged

in the form of pointers and network and in relational model the data was arranged in the form of tables.

The data stored in the form tables is easy to stored, maintain and understand. Many DBMS has been

developed using approach of hierarchical and network models. Any DBMS that uses the relational data

model for data storage and modeling Is called RDBMS. In RDBMS we can create relations among tables

and can access the information from tables - while tables store stored in separately file and may or may

not have identical structures. The RDBMS is based upon the rules given by Dr. Codd known as Dr. Codds

Rules.

HDBMS (Heterogeneous DBMS)

In RDBMS we store the information related to the same kind of data like student data, teacher data,

employee data etc. In HDBMS we store the data in the database which is entirely different.

DDBMS (Distributed DBMS)

During 1950s & 1960s there was trend to use independent or decentralized system. There was a

duplication of hardware and facilities. In a centralized database system, the DBMS & data reside at a

single place and all the control & location is limited to a single location, but the PCs are distributed

geographically. Distributed system is parallel computing using multiple independent computers

communicating over a network to accomplish a common objective or task. The type of hardware,

programming languages, operating systems and other resources may vary drastically. It is similar to

computer clustering with the main difference being a wide geographic dispersion of the resources

For example an organization may have an office in a building and have many sub- buildings that are

connected using LAN. The current trend is towards distributed systems. This is a centralized system

connected to intelligent remote sites. Each remote site have own storage and processing capabilities -but in a centralized or network there is a single storage.

OODBMS (Object Oriented DBMS)

Object-Oriented Database Management Systems (OODBMSs) have been developed to support new

kinds of applications for which semantic and content are represented more efficiently with the object

model. Therefore, the OODBMSs present the two main problems:

Impedance mismatch:It is basically due to two reasons. Firstly, the no suitable abstractions of the

operating systems, so when a client object has to invoke a method that is offered by a server object, and

both objects are not into the same address space, it is necessary to use the mechanisms that are offeredby the operating system, and these mechanisms do not became proper to the object oriented paradigm

since they are oriented to communicate processes. In order to solve this problem intermediate software

is included (e.g. COM or CORBA).In the second place, an impedance mismatch is also caused every time

that the object-oriented applications need to use the operating system services.

Interoperability problem between object models:Although different system elements use the

object-oriented paradigm, an interoperability problem can exist between them. So, an application


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implemented using the C++ language, with the C++ object model, can easily interact with its objects, but

when it wants to use objects that have been created with another programming language or another

object-oriented database an interoperability problem appears.

The programming LANGUAGES like C, FORTRAN, PASCAL & FORTRAN use the POP (Procedure

Oriented Approach) to develop applications, but the current trend is towards OOP (Object Oriented

Programming). The languages like C++, Java, Oracle, C# (C Sharp). Visual Basic 6 use this approach. Many

databases have been developed that follows this approach (OI approach) like Oracle. So the DBMS

which follow OOP approach is called OODBMS.

Q. 14. Explain the difference between physical and logical data independence.

Ans. One of the biggest advantages of database is data independence. It means we can change the

conceptual schema at one level without affecting the data at other level. It means we can change the

structure of a database without affecting the data required by users and program. This feature was not

available in file oriented approach. There are two types of data independence and they are:

1. Physical data independence

2. Logical data independence

Data Independence The ability to modify schema definition in on level without affecting schema

definition in the next higher level is called data independence. There are two levels of dataindependence:

1. Physical data independence is the ability to modify the physical schema without

causing application programs to be rewritten. Modifications at the physical level are occasionally

necessary to improve performance. It means we change the physical storage/level without affecting the

conceptual or external view of the data. The new changes are absorbed by mapping techniques.

2. Logical data independence in the ability to modify the logical schema without

causing application program to be rewritten. Modifications at the logical level are necessary wheneverthe logical structure of the database is altered (for example, when money-market accounts are added to

banking system).

Logical Data independence means if we add some new columns or remove some columns from table

then the user view and programs should not changes. It is called the logical independence. For example:

consider two users A & B. Both are selecting the empno and ename. If user B add a new column salary in


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his view/table then it will not effect the external view user; user A, but internal view of database has

been changed for both users A & B. Now user A can also print the salary.

User As External View

(View before adding a new column)

User Bs external view

(View after adding a new column salary)

It means if we change in view then program which use this view need not to be changed.

Logical data independence is more difficult to achieve than is physical data independence, since

application programs are heavily dependent on the logical structure of the data that they access.

Logical data independence means we change the physical storage/level without effecting the

conceptual or external view of the data. Mapping techniques absorbs the new changes.

Q. 15. What is physical data independence?

Ans.Physical data independence is the ability to modify the physical schema without causing

application programs to be rewritten. Modifications at the physical level are occasionally necessary to

improve performance. It means we change the physical storage/level without affecting the conceptual

or external view of the data. The new changes are absorbed by mapping techniques.


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Q. 16. What do you mean by data redundancy?

Ans. Redundancy is unnecessary duplication of data. For example if accounts department and

registration department both keep student name, number and address.

Redundancy wastes space and duplicates effort in maintaining the data.

Redundancy also leads to inconsistency.

Inconsistent data is data which contradicts itself - e.g. two different addresses for a given student

number. Inconsistency cannot occur if data is represented by a single entry (i.e. if there is no

redundancy).

Controlled redundancy

Some redundancy may be desirable (for efficiency). A DBMS should be aware of it, and take care of

propagating updates to all copies of a data item.

This is an objective, not yet currently supported.

Q. 17. What do you man by database schema?

Ans.It is necessary to describe the organization, of the data in a formal manner. The logical and

physical database descriptions are used by DBMS software. The complete and overall description of data

is referred to as schema, The schema and subschema words are brought into DBMS by CODASYL

(Conference on data system language1 committee) and also by the CODASYLs database task group.

Schema is also referred to as conceptual model or global view (community view) of data. Suppose a

complete description of collected data having all classes and student data4 all employees (teaching &non-teaching) data and other concept of data related to the college is called Schema of the college. We

can say that we relate whole college data logically, which is called schema.


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Q. 18. Explain the distinctions among the terms primary key, candidate key and superkey.

Or

What is the significance of foreign key? Or What are the various keys?

Ans.Keys: As there are number of keys can be defined, but some commonly and mainly used keys

are explained as below:

1. Primary Key

A key is a single attribute or combination of two or more, attributes of an entity that is used to

identify one or more instances of the set. The attribute Roll # uniquely identifies an instance of the

entity set STUDENT. It tells about student Amrita having address 101, Kashmir Avenue and phone no.

112746 and have paid fees 1500 on basis of Roll No. 15. The 15 is unique value and it gives unique

identification of students So here Roll No is unique attribute and such a unique entity identifies called

Primary Key. Primary key cannot be duplicate.

From the definition of candidate key, it should be clear that each relation must have at least one

candidate key even if it is the combination of all the attributes in the relation since all tuples in a relation

are distinct. Some relations may have more t one candidate keys.

As discussed earlier, the primary key of a relation is an arbitrarily but permanently selected

candidate key. The primary key is important since it is the sole identifier for the tuples in a relation. Any

tuple in a database may be identified by specifying relation name, primary key and its value. Also for a

tuple to exist in a relation, it must be identifiable and therefore it must have a primary key. The

relational data model therefore imposes the following two integrity constraints:

(a) No component of a primary key value can be null;

(b) Attempts to change the value of a primary key must be carefully controlled.

The first constraint is necessary because if we want to store information about some entity, then we

must be able to identify it, otherwise difficulties are likely to arise. For example, if a relation

CLASS (STUNO, LECTURER, CNO)

has (STUNO, LECTURER) as the primary key then allowing tuples like

3123 NULL CP302

NULL SMITH CP302

is going to lead to ambiguity since the two tuples above may or may not be identical and the integrity

of the database may be compromised. Unfortunately most commercial database systems do not


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support the concept of primary key and it would be possible to have a database state when integrity of

the database is violated.

The second constraint above deals with changing of primary key values. Since the primary key is the

tuple identifier, changing it needs very careful controls. Codd has suggested three possible approaches:

Method 1

Only a select group of users be authorised to change primary key values.

Method 2

Updates on primary key values be banned. If it was necessary to change a primary key, the tuple

would first be deleted and then a new tuple with new primary key value but same other values would be

inserted. Of course, this does require that the old values of attributes be remembered and be reinserted

in the database.

Method 3

A different command for updating primary keys be made available. Making a distinction in altering

the primary key and another attribute of a relation would remind users that care needs to be taken in

updating primary keys.

2. Secondary Key

The ke1 which is not giving the unique identification and have duplicate infonna6o is called

secondary key, e g in a STUDENT entity if Roll Number is the primary key, then Name of the student,

address of the student, Phone number of the student and the fees paid by the student all are secondary

keys. A secondary key is an attribute or combination of attributes that not be primary key and haveduplicate data. In otherworlds secondary key is used after the identification of the primary key. Also we

can identify the data from the combination of the secondary keys.

3. Super Key

If we add additional attributes to a primary key, the resulting combination would still uniquely

identify an instance of the entity set Such keys are called super keys A primary key is therefore a

minimum super key For example, if DOB (date of birth field or attribute) is the primary key, then by

adding some additional information about the day of the month key in the DOB field, this field or

attribute becomes more powerful and useful Such type of key is called super key Super key are less used

in a small database file. Now these days it has less importance, but due to its feature, this key gives thecomplete description of the database.

4. Candidate Key

There may be two or more attributes or combination of attributes that uniquely identify an instance

of an entity set These attributes or combination of attributes are called candidate keys. Candidate key

also gives unique identification. Candidate key comes with primary key. A candidate is a combination of


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two or more attributes e.g. if Roll No. and student name are two different attributes then we combine

these two attribute and form a single attribute Roll No. & Name, then this combination is the candidate

key and it is unique and gives unique identification about a particular roll no. and about particular name.

5. Alternative Key

A candidate key which is not the primary key is called alternative key, e.g. if Roll No. and Name

combination is the candidate key, then if Roll No, is the primary key, other key in the candidate key is

Name. Name attribute work as the alternative key.

6 Foreign Key

Suppose there are some relations as: SP (S#, P#, QTY), relation S (S#, S Name, status, city) and

relation P (P#, PName, Color, Weight, City). We know entity SP is defined as the relationship of the

relation S and the relation P. These two relations has sand P# as the Primary Keys in relation S and P

respectively, but in the relation SP we can take either # as the primary key or P# as the primary key.

Suppose if we take P# as the primary key, then other primary key S# which is actually the primary key,

but do not work as primary key in the relation SF is called the Foreign Key. If S# is the primary key then

P# is the Foreign Key. Similarly in the relation ASSIGNMENT, attribute Emp #, Prod #, Job # are given and

if S# and P# are the primary keys, then the Job # key is the Foreign Keys.

Q. 19. What are the major functions of a database administrator?

Ans.RESPONSIBILITIES OF DBA

As we know DBA is the overall commander of a computer system, so it has number of duties, but

some of his/her major responsibilities are as follows:

1. DBA can control the data, hardware, and software and gives the instructions to the application

programmer, end user and naive user.

2. DBA decides the information contents of the database. He decides the suitable database file

structure for arrangement of data. He/She uses the proper DDL techniques.

3. DBA compiles the whole data in a particular order and sequence.

4. DBA decides where data can be stored i.e. take decision about the storage structure.

5. DBA decides which access strategy and technique should be used for accessing the data.

6. DBA communicates with the user by appropriate meeting. DBA co-operates with user.


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7. DBA also define and apply authorized checks and validation procedures.

8. DBA also takes backup of the data on a backup storage device so that if data can then lost then it

can be again recovered and compiled. DBA also recovers the damaged data.

9. DBA also changes the environment according to user or industry requirement and monitor the

performance.

10. DBA should be good decision-maker. The decision taken by DBA should be correct, accurate &

efficient.

11. DBA should have leadership quality.

12. DBA liaise with the user in the business to take confidence of the customer about the availability

of data.

Q. 20. What do you mean by relationships? Explain different types of relationships.

Ans.Relationships: One table (relation) may be linked with another in what is known as

a relationship. Relationships may be built into the database structure to facilitate the operation of

relational joins at runtime.

1.

A relationship is between two tables in what is known as a one-to-many or parent-child or master-detail relationship where an occurrence on the one or parent or master table

may have any number of associated occurrences on the many or child or detail table. To

achieve this, the child table must contain fields which link back the primary key on

the parent table. These fields on the child table are known as a foreign key, and the parent table

is referred to as the foreign table (from the viewpoint of the child).

2.

It is possible for a record on the parent table to exist without corresponding records on

the child table, but it should not be possible for an entry on the child table to exist without a

corresponding entry on the parent table.

3.

A child record without a corresponding parent record is known as an orphan.

4.

It is possible for a table to be related to itself. For this to be possible it needs a foreign key which

points back to the primary key. Note that these two keys cannot be comprised of exactly thesame fields otherwise the record could only ever point to itself.

5.

A table may be the subject of any number of relationships, and it may be the

parent in some and the child in others.

1.

Some database engines allow a parent table to be linked via a candidate key, but if this were

changed it could result in the link to the child table being broken.


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2.

Some database engines allow relationships to be managed by rules known as referential

integrity or foreign key restraints. These will prevent entries onchild tables from being created if

the foreign key does not exist on the parent table, or will deal with entries on child tables when

the entry on the parent table is updated or deleted.

Relational Joins

The join operator is used to combine data from two or more relations (tables) in order to satisfy a

particular query. Two relations may be joined when they share at least one common attribute. The join

is implemented by considering each row in an instance of each relation. A row in relation R1 is joined to

a row in relation R2 when the value of the common attribute(s) is equal in the two relations. The join of

two relations is often called a binary join.

The join of two relations creates a new relation. The notation R1 x R2 indicates the join of relations

R1 and R2. For example, consider the following:

Note that the instances of relation RI and R2 contain the same data values for attribute B. Data

normalisation is concerned with decomposing a relation (e.g. R(A,B,C,D,E) into smaller relations (e.g. R1

and R2). The data values for attribute B in this context will be identical in R1 and R2. The instances of R1

and R2 are projections of

the instances of R(A,B,C,D,E) onto the attributes (A,B,C) and (B,D,E) respectively. A projection will not

eliminate data values duplicate rows are removed, but this will not remove a data value from any

attribute.

The join of relations RI and R2 is possible because B is a common attribute. The result of the join is:


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The row (2 4 5 7 4) was formed by joining the row (2 4 5) from relation R1 to the row (4 7 4) from

relation R2. The two rows were joined since each contained the same value for the common attribute B.

The row (2 4 5) was not joined to the row (6 2 3) since the values of the common attribute (4 and 6) are

not the same.

The relations joined in the preceding example shared exactly one common attribute. However,relations may share multiple common attributes. All of these common attributes must be used in

creating a join. For example, the instances of relations R1 and R2 in the following example are joined

using the common attributes B and C:

Before the join:


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After the join:

The row (6 1 4 9) was formed by joining the row (6 1 4) from relation R1 to the row

(1 4 9) from relation R2. The join was created since the common set of attributes (B and

C) contained identical values (1 and 4). The row (6 1 4) from R1 was not joined to the

row (1 2 1) from R2 since the common attributes did not share identical values - (1 4) in

R1 and (1 2) in R2.

The join operation provides a method for reconstructing a relation that was decomposed into two

relations during the normalisation process. The join of two rows, however, can create a new row that

was not a member of the original relation. Thus invalid information can be created during the join

process.


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Now suppose that a list of courses with their corresponding room numbers is required. Relations R1

and R4 contain the necessary information and can be joined using the attribute HOUR. The result of this

join is:


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This join creates the following invalid information (denoted by the coloured rows):

Smith, Jones, and Brown take the same class at the same time from two different instructors in twodifferent rooms.

Jenkins (the Maths teacher) teaches English.

Goldman (the English teacher) teaches Maths.

Both instructors teach different courses at the same time.

Another possibility for a join is R3 and R4 (joined on INSTRUCTOR). The result would be:

This join creates the following invalid information:

Jenkins teaches Math I and Algebra simultaneously at both 8:00 and 9:00.


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A correct sequence is to join R1 and R3 (using COURSE) and then join the resulting relation with R4

(using both INSTRUCTOR and HOUR). The result would be:

Extracting the COURSE and ROOM attributes (and eliminating the duplicate row produced for the

English course) would yield the desired result:

The correct result is obtained since the sequence (R1 x r3) x R4 satisfies the lossless (gainless?) join

property

A relational database is in 4th normal form when the lossless join property can be used to answer

unanticipated queries. However, the choice of joins must be evaluated carefully. Many differentsequences of joins will recreate an instance of a relation. Some sequences are more desirable since they

result in the creation of less invalid data during the join operation.

Suppose that a relation is decomposed using functional dependencies and multi- valued

dependencies. Then at least one sequence of joins on the resulting relations exists that recreates the

original instance with no invalid data created during any of the join operations.


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For example, suppose that a list of grades by room number is desired. This question, which was

probably not anticipated during database design, can be answered without creating invalid data by

either of the following two join sequences:

The required information is contained with relations R2 and R4, but these relations

cannot be joined directly. In this case the solution requires joining all 4 relations.

The database may require a lossless join relation, which is constructed to assure that any ad hoc

inquiry can be answered with relational operators. This relation may contain attributes that are not

logically related to each other. This occurs because the relation must serve as a bridge between the

other relations in the database. For example, the lossless join relation will contain all attributes that

appear only on the left side of a functional dependency. Other attributes may also be required,

however, in developing the lossless join relation.

Consider relational schema R (A, B, C, D), A B and C D. Relations and are

in 4th normal form. A third relation however, is required to satisfy the lossless join property.

This relation can be used to join attributes B and D. This is accomplished by joining relations R1 and R3

and then joining the result to relation

R2. No invalid data is created during these joins. The relation is the lossless join relation for

this database design.

A relation is usually developed by combining attributes about a particular subject or entity. The

lossless join relation, however, is developed to represent a relationship among various relations. The

lossless join relation may be difficult to populate initially and difficult to maintain - a result of including

attributes that are not logically associated with each other.

The attributes within a lossless join relation often contain multi-valued dependencies. Consideration

of 4th normal form is important in this situation. The lossless join relation can sometimes be

decomposed into smaller relations by eliminating the multi-valued dependencies. These smaller

relations are easier to populate and maintain.


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Q. 21. What is an ER-diagram? Construct an ER diagram for a hospital with a set of patients and a

set of doctors. Associate with each patient a log o1 the various tests and examinations conducted.

Or

Discuss in detail the ER diagram.

Or

What is one to many relationship? Give examples.

Or

Draw an ER diagram for a library management system, make suitable assumptions. Describe

various symbols used in ER. diagram.

Or

Construct an ER diagram for a university registrars office. The office maintains data about each

class, including the instructor, the enrollment and the time and place of the class meetings. For each

student class pair, a grade is recorded also design a relational database for the said I.R. diagram.

Ans.E-R model grew out of the exercise of using commercially available DBMS to model application

database. Earlier DBMS were based on hierarchical and network approach. E-R is a generalization of

these models. Although it has some means of describing the physical database model, it is basicallyuseful in the design of logical database model. This analysis is then used to organize data as a relation,

normalizing relations and finally obtaining a relational database model.

The entity-relationship model for data uses three features to describe data. These are:

1. Entities, which specify distinct real-world items in an application.

2. Relationships, which connect entities and represent meaningful dependencies

between them.

3. Attributes, which specify properties of entities and relationships.

We illustrate these terms with an example. A vendor supplying items to a company, for example, is

an entity. The item he supplies is another entity. A vendor supplying items are related in the sense that a

vendor supplies an item. The act of supplying defines a relationship between a vendor and an item. An

entity set is a collection of similar entities. We can thus define a vendor set and an item set. Each


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member of an entity set is described by some attributes. For example, a vendor may be described by the

attributes:

(vendor code, vendor name, address)

An item may be described by the attributes:

(item code, item name)

Relationship also can be characterized by a number of attributes. We can think of the relationship as

supply between vendor and item entities: The relationship supply can be described by the attributes:

(order no. date of supply)


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Relationship between Entity Sets

The relationship between entity sets may be many-to-many (M: N), one-to-many (1: M), many-to-

one (M: 1) or one-to-one (1:1). The 1:1 relationship between entity sets E1 and E2 indicates that for

each entity in either set there is at most one entity in the second set that is associated with it. The 1: M

relationship from entity set E1 to E2 indicates that for an occurrence of the entity from the set E1, there

could be zero, one or more entities from the entity set E2 associated with it. Each entity in E2 is

associated with at most one entity in the entity set E1. In the M: N relationship between entity sets

E1 and E2, there is no restriction to the number of entities in one set associated with an entity in theother set. The database structure, employing the E-R model is usually shown pictorially using entity-

relationship (E-R) diagram.

To illustrate these different types of relationships consider the following entity sets: DEPARTMENT,

MANAGER, EMPLOYEE, and PROJECT


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The relationship between a DEPARTMENT and a MANAGER is usually one-to- one; there is only one

manager per department and a manager manages only one department. This relationship between

entities is shown in Figure. Each entity is represented by a rectangle and the relationship between them

is indicated by a direct line. The relationship for MANAGER to DEPARTMENT and from DEPARTMENT to

MANAGER is both 1:1. Note that a one-to-one relationship between two entity sets does not imply that

for an occurrence of an entity from one set at any time there must be an occurrence of an entity in the

other set. In the case of an organization, there could be times when a department is without a manager

or when an employee who is classified as a manager may be without a department to manage. Figure

shows some instances of one-to-one relationships between the entities DEPARTMENT and MANAGER.

A one-to-many relationship exists from the entity MANAGER to the entity EMPLOYEE because there

are several employees reporting to the manager. As we just pointed out, there could be an occurrence

of the entity type MANAGER having zero occurrences of the entity type EMPLOYEE reporting to him or

her. A reverse relationship, from EMPLOYEE to MANAGER, would be many to one, since many

employees may be supervised by a single manager. However, given an instance of the entity set

EMPLOYEE, there could be only one instance of the entity set MANAGER to whom that employee

reports (assuming that no employee reports to more than one manager). The relationship between

entities is illustrated in Figures shows some instances of this relationship.

Figure: 1:M Relationship


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Figure: Instances of 1: M Relationship

The relationship between the entity EMPLOYEE and the entity PROJECT can be derived as follows:

Each employee could be involved in a number of different projects, and a number of employees couldbe working on a given project. This relationship between EMPLOYEE and PROJECT is many-to-many. It is

illustrated in Figures shows some instances of such a relationship.

Figure: M : N Relationship

Figure: Instances of M:N Relationship

In the entity-relationship (E-R) diagram, entities are represented by rectangles, relationships by a

diamond-shaped box and attributes by ellipses or ovals. The following

E-R diagram for vendor, item and t

DBMS Disadvantages

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