Relational Approach

8/10/2019 Relational Approach

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Relational Approach


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Basic Structure

Given sets A1, A2, . An a relation r is a subset of A1 x A2 x x An Thus a relation is a set of n-tuples ( a 1, a 2, , a n) wherea i A i

Example: ifcustomer-name = {Jones, Smith, Curry, Lindsay}customer-street = {Main, North, Park}customer-city = {Harrison, Rye, Pittsfield}

Then r = {(Jones, Main, Harrison), (Smith, North, Rye),(Curry, North, Rye), (Lindsay, Park, Pittsfield)} is a relationover customer-name x customer-street x customer-city


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Relation Schema

A1, A2, , An are attributes R = ( A1, A2, , An ) is a relation schema

Customer-schema - (customer-name,customer-street,customer-city ) r (R ) is a relation on the relation schema R

customer (Customer-schema)


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Relation Instance

The current values (relation instance)of a relation are specified by a table

An element t of r is a tuple,represented by a row in a table

JonesSmithCurry

Lindsay

customer-name

MainNorthNorthPark

customer-street

HarrisonRyeRye

Pittsfield

customer-city

customer


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Keys Let K R K is a superkey of R if values for K are sufficient to

identify a unique tuple of each possible relation r(R) bypossible r we mean a relation r that could exist in the

enterprise we are modeling.Example: { customer-name, customer-street } and{customer-name } are both superkeys of Customer , if no

two customers can possibly have the same name.

K is a candidate key if K is minimalExample: { customer-name } is a candidate key forCustomer, since it is a superkey {assuming no twocustomers can possibly have the same name), and nosubset of it is a superkey.


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Determining Keys from E-RSets

Strong entity set . The primary key of the entity setbecomes the primary key of the relation.

Weak entity set . The primary key of the relationconsists of the union of the primary key of the strongentity set and the discriminator of the weak entity set.

Relationship set . The union of the primary keys of therelated entity sets becomes a super key of the relation.For binary many-to-one relationship sets, the primarykey of the many entity set becomes the relationsprimary key.For one-to- one relationship sets, the relations primarykey can be that of either entity set.


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Data Definition Language (DDL)

The schema for each relation. The domain of values associated with each

attribute. Integrity constraints The set of indices to be maintained for each

relations. Security and authorization information for each

relation. The physical storage structure of each relation

on disk.

Allows the specification of not only a set of relations but alsoinformation about each relation, including:


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Domain Types in SQL char(n). Fixed length character string, with user-

specified length n. varchar(n). Variable length character strings, with user-

specified maximum length n. int. Integer (a finite subset of the integers that is

machine-dependent). smallint. Small integer (a machine-dependent subset of

the integer domain type). numeric(p,d). Fixed point number, with user-specified

precision of p digits, with n digits to the right of decimalpoint.


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Domain Types in SQL (Cont.) real, double precision. Floating point and double-precision

floating point numbers, with machine-dependent precision. float(n). Floating point number, with user-specified precision

of at least n digits. date. Dates, containing a (4 digit) year, month and date. time. Time of day, in hours, minutes and seconds.

Null values are allowed in all the domain types. Declaringan attribute to be not null prohibits null values for that

attribute. create domain construct in SQL-92 creates user-defined

domain typescreate domain person-name char (20) not null


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Built-in Data Types in SQL date: Dates, containing a (4 digit) year, month and date

Example: date 2005 -7-27 time: Time of day, in hours, minutes and seconds.

Example: time 09:00:30 time 09:00:30.75 timestamp : date plus time of day

Example: timestamp 2005 -7-27 09:00:30.75 interval: period of time

Example: interval 1 day Subtracting a date/time/timestamp value from another gives

an interval value Interval values can be added to date/time/timestamp values


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Build-in Data Types in SQL(Cont.)

Can extract values of individual fields fromdate/time/timestamp Example: extract (year from r.starttime)

Can cast string types to date/time/timestamp Example: cast as

date Example: cast as

time


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User-Defined Types create type construct in SQL creates user-defined type

create type Dollars as numeric (12,2) final

create domain construct in SQL-92 creates user-defined

domain types

create domain person_name char (20) not null

Types and domains are similar. Domains can haveconstraints, such as not null , specified on them.


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Large-Object Types

Large objects (photos, videos, CAD files,etc.) are stored as a large object : blob : binary large object -- object is a large

collection of uninterpreted binary data (whoseinterpretation is left to an application outsideof the database system)

clob : character large object -- object is a large

collection of character data When a query returns a large object, a pointer

is returned rather than the large object itself.


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Create Table Construct An SQL relation is defined using the create tablecommand:

create table r ( A1 D1, A2 D2, ..., An Dn,(integrity-constraint 1),...,

(integrity-constraint k)) r is the name of the relation each A i is an attribute name in the schema of relation r Di is the data type of values in the domain of attribute

A i Example:

create table branch (branch-name char(15) not null,branch-city char(30),assets integer)


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Drop and Alter Table Constructs The drop table command deletes all information about the

dropped relation from the database. The after table command is used to add attributes to an

existing relation. All tuples in the relation are assigned null

as the value for the new attribute. The form of the altertable command isalter table r add A D

where A is the name of the attribute to be added to relation rand D is the domain of A.

The alter table command can also be used to dropattributes of a relation

alter table r drop A when A is the name of an attribute of relation r .


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Integrity Constraints in Create Table not null

primary key ( A1, ..., An ) check (P,) where P is a predicate

Example: Declare branch-name as the primary key for branch and ensure that the values of assets are non-negative.

create table branch(branch-name char(15) not null,branch-city char(30)assets integer,primary key (branch-name),

check (assets > - 0))


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Entity integrity In a relational database , entity integrity is a property that

ensures that no records are duplicated and that no attributes that make up the primary key are NULL. It is one of theproperties necessary to ensure the consistency of thedatabase.

Entity Integrity ensures that there are no duplicate records

within the table and that the field that identifies each recordwithin the table is unique and never null. The existence of the Primary Key is the core of the entity

integrity. If you define a primary key for each entity, theyfollow the entity integrity rule.

Entity integrity specifies that the Primary Keys on everyinstance of an entity must be kept, must be unique and musthave values other than NULL.
http://en.wikipedia.org/wiki/Relational_databasehttp://en.wikipedia.org/wiki/Attributehttp://en.wikipedia.org/wiki/Primary_keyhttp://en.wikipedia.org/wiki/Null_%28SQL%29http://en.wikipedia.org/wiki/Null_%28SQL%29http://en.wikipedia.org/wiki/Primary_keyhttp://en.wikipedia.org/wiki/Attributehttp://en.wikipedia.org/wiki/Relational_database


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Entity integrity Although most relational databases do not specifically dictate that a

table needs to have a Primary Key, it is good practice to design aPrimary Key for each table in the relational model. This mandates noNULL content, so that every row in a table must have a value thatdenotes the row as a unique element of the entity.

Entity Integrity is the mechanism the system provides to maintainprimary keys. The primary key serves as a unique identifier for rows

in the table. Entity Integrity ensures two properties for primary keys: The primary key for a row is unique; it does not match the primary keyof any other row in the table. The primary key is not null, no componentof the primary key may be set to null. The uniqueness property ensuresthat the primary key of each row uniquely identifies it; there are noduplicates. The second property ensures that the primary key hasmeaning, has a value; no component of the key is missing.

The system enforces Entity Integrity by not allowing operations(INSERT, UPDATE) to produce an invalid primary key. Anyoperation that creates a duplicate primary key or one containingnulls is rejected.


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Integrity Constraints Integrity constraints guard against accidental

damage to the database, by ensuring thatauthorized changes to the database do notresult in a loss of data consistency. A checking account must have a balance

greater than $10,000.00 A salary of a bank employee must be at

least $4.00 an hour A customer must have a (non-null) phone

number


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Not Null Constraint

Declare branch_name for branch is not null branch_name char (15) not null

Declare the domain Dollars to be not null

create domain Dollars numeric (12,2) notnull

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The Unique Constraint

unique ( A1, A2, , Am) The unique specification states that theattributes

A1, A2, Amform a candidate key. Candidate keys are permitted to be null

(in contrast to primary keys).

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The check clause check (P ), where P is a predicate

Example: Declare branch_name as the primary key forbranch and ensure that the values of assets are non-negative.

create table branch(branch_name char (15) ,branch_city char (30),assets integer ,primary key (branch_name ),check (assets >= 0))


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The check clause (Cont.) The check clause in SQL-92 permits domains to be

restricted: Use check clause to ensure that an hourly_wage

domain allows only values greater than a specifiedvalue.

create domain hourly_wage numeric(5,2)constraint value_test check (value > =

4.00) The domain has a constraint that ensures that the

hourly_wage is greater than 4.00 The clause constraint value_test is optional;

useful to indicate which constraint an updateviolated.


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Referential integrity Referential integrity in a relational database is consistency

between coupled tables. Referential integrity is usually enforced bythe combination of a primary key or candidate key (alternate key )and a foreign key . For referential integrity to hold, any field in a table that is declared a foreign key can contain only values from a parenttable's primary key or a candidate key. For instance, deleting arecord that contains a value referred to by a foreign key in anothertable would break referential integrity. The relational databasemanagement system (RDBMS) enforces referential integrity,normally either by deleting the foreign key rows as well to maintainintegrity, or by returning an error and not performing the delete.Which method is used would be defined by the definition of thereferential integrity constraint. Example

An employee database stores the department in which eachemployee works. The field "DepartmentNumber" in the Employeetable is declared a foreign key, and it refers to the field "Index" in theDepartment table which is declared a primary key. Referentialintegrity would be broken by deleting a department from theDepartment table if employees listed in the Employee table arelisted as working for that department.
http://en.wikipedia.org/wiki/Relational_databasehttp://en.wikipedia.org/wiki/Primary_keyhttp://en.wikipedia.org/wiki/Candidate_keyhttp://en.wikipedia.org/wiki/Alternate_keyhttp://en.wikipedia.org/wiki/Foreign_keyhttp://en.wikipedia.org/wiki/Table_%28database%29http://en.wikipedia.org/wiki/Relational_database_management_systemhttp://en.wikipedia.org/wiki/Relational_database_management_systemhttp://en.wikipedia.org/wiki/Databasehttp://en.wikipedia.org/wiki/Databasehttp://en.wikipedia.org/wiki/Relational_database_management_systemhttp://en.wikipedia.org/wiki/Relational_database_management_systemhttp://en.wikipedia.org/wiki/Table_%28database%29http://en.wikipedia.org/wiki/Foreign_keyhttp://en.wikipedia.org/wiki/Alternate_keyhttp://en.wikipedia.org/wiki/Candidate_keyhttp://en.wikipedia.org/wiki/Primary_keyhttp://en.wikipedia.org/wiki/Relational_database


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Referential Integrity Ensures that a value that appears in one relation for a given set

of attributes also appears for a certain set of attributes in anotherrelation. Example: If Perryridge is a branch name appearing in one

of the tuples in the account relation, then there exists a tuplein the branch relation for branch Perryridge.

Primary and candidate keys and foreign keys can be specified aspart of the SQL create table statement: The primary key clause lists attributes that comprise the

primary key. The unique key clause lists attributes that comprise a

candidate key. The foreign key clause lists the attributes that comprise the

foreign key and the name of the relation referenced by theforeign key. By default, a foreign key references the primarykey attributes of the referenced table.

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Referential Integrity in SQL Example

create table customer(customer_name char (20) ,customer_street char (30),customer_city char (30),primary key (customer_name ))

create table branch(branch_name char (15) ,branch_city char (30),assets numeric (12,2),primary key (branch_name ))


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Referential Integrity in SQL Example (Cont.)

create table account(account_number char (10) ,branch_name char (15),balance integer ,primary key (account_number),foreign key (branch_name ) references branch )

create table depositor(customer_name char (20) ,account_number char (10) ,

primary key (customer_name, account_number),foreign key (account_number ) referencesaccount,foreign key (customer_name ) referencescustomer )

Relational Approach

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