Concepts of Database Management, Fifth Edition Chapter 4: The Relational Model 3: Advanced Topics.

Concepts of Database Concepts of Database Management, Fifth Management, Fifth

EditionEdition

Chapter 4: Chapter 4:

The Relational Model 3:The Relational Model 3:

Advanced Topics Advanced Topics

2Concepts of Database Management, 5th Edition

Objectives

Define, describe, and use views

Use indexes to improve database performance

Examine the security features of a database management system (DBMS)

Discuss entity, referential, and legal-values integrity

Make changes to the structure of a relational database

Define and use the system catalog


Views

Application program’s or individual user’s picture of the database

Less involved than full database

Offers simplification

Provides measure of security

Sensitive tables or columns omitted where not appropriate


SELECT Command

Called the defining query

Indicates precisely what to include in the view

Query acts as a sort of window into the database

Does not produce a new table, only the view of the table


Figure 4.1: SQL to Create View

CREATE VIEW Housewares ASSELECT PartNum, Description, OnHand, PriceFROM PartWHERE Class=‘HW’;


Query on a View With a query that involves a view, the DBMS

changes the query to one that selects data from table(s) in the database that created the view

The DBMS merges the query with the query that defines the view to form the query that is actually executed

One advantage of this approach is that the view never exists in its own right so any update to the table is immediately available in the view

If the view were a table, this would not be the case


Query on a View Selects data only from Tables created in the

view

Query is merged with query used to create view

SELECT *FROM HousewaresWHERE OnHand< 25;

SELECT PartNum, Description, OnHand, PriceFROM PartWHERE Class=‘HW’AND OnHand< 25;

Actually executes as


Figures 4.3 - 4.4: Access Query Design of View


Access Query Design View with Changed Field Names

SQL can be used to change the field names in a view by including the new field names in the CREATE VIEW statement

The CREATE VIEW statement would be:CREATE VIEW SalesCust (Snum, SLast, SFirst, Cnum, CName) ASSELECT Rep.RepNum, LastName, FirstName, CustomerNum, CustomerNameFROM Rep, CustomerWHERE Rep.RepNum=Customer.RepNum;


Figures 4.5-4.6: Access Query Design of View with Changed Field Names


Row and Column Subset View

Consists of a subset of the rows and columns in some individual table

Because the query can be any SQL query, a view could also join two or more tables


Advantages of Views

Provides data independence

Same data viewed by different users in different ways

Contains only information required by a given user


Indexes

Conceptually similar to book index

Increases data retrieval efficiency

Automatically assigns record numbers

Used by DBMS, not by users

Fields on which index built called Index Key


Figure 4.10: Customer Table with Record Numbers


Figure 4.11: Customer Table Index on CustomerNum


Figure 4.12: Table Indexes on CreditLimit, RepNum


Pros/Cons of Indexes

Can be added or dropped without loss of function

Can make retrieval more efficient

Occupies space that might be required for other functions

DBMS must update index whenever corresponding data are updated


SQL to Create Index

CREATE INDEX CustomerNameON Customer (CustomerName);


Creating Indexes

Single-field index – an index whose key is a single field

Multiple-field index

An index with more than one key field

List the most important key first

If data for either key appears in descending order, follow the field name with the letters DESC


SQL to Delete Index

DROP INDEX RepBal;


Figure 4.13: Index on Single Field in Access


Figure 4.14: Index on Multiple Fields in Access


Security

Prevention of unauthorized access to database

Two SQL security mechanisms

GRANT provides privileges to users

REVOKE removes privileges from usersGRANT SELECT ON Customer TO JONES;

REVOKE SELECT ON Customer FROM JONES;


Integrity Rules

Related to foreign keys and primary keys

Defined by Dr. E.F. Codd

Entity integrity

No field that is part of the primary key may accept null values


Integrity Rules (con’t)

To specify primary key, enter a PRIMARY KEY clause in either an ALTER TABLE or a CREATE TABLE command

Foreign key – a field (or collection of fields) in a table whose value is required to match the value of the primary key for a second table


Figure 4.15: Primary Key in Access

PRIMARY KEY (CustomerNum)


Figure 4.16: Multi-Field Primary Key in Access

PRIMARY KEY (OrderNum, PartNum)


Referential integrity

If Table A contains a foreign key matching the primary key of Table B, then values must match for some row in Table B or be null

Usually a foreign key is in a different table from the primary key it is required to match

The only restriction is that the foreign key must have a name that is different from the primary key because the fields are in the same table


Figure 4.17: Relationships Window to Relate Tables in Access

FOREIGN KEY (RepNum) REFERENCES Rep


Cascade Delete and Update

Cascade delete - ensures that the deletion of a master record deletes all records in sub tables related to it

Cascade update – ensures that changes made to the primary key of the master table are also made in the related records


Figure 4.18: Specifying Referential Integrity


Enforcing Referential Integrity

With referential integrity enforced, users are not allowed to enter a record that does not match any sales rep currently in the Rep table

An error message, such as the one shown in Figure 4.19, appears when an attempt is made to enter an invalid record


Figure 4.19: Violating Referential Integrity on Adding


Legal-Values Integrity

States no record can exist with field values other than legal ones

Use SQL CHECK clause

Validation rule – in Access, a rule that data entered into a field must follow

Validation – in Access, text to inform the user of the reason for the rejection when the user attempts to enter data that violates the rule

CHECK (CreditLimit IN (5000, 7500, 10000, 15000)) ;


Validation Rule in Access


Structure Changes Can change the database structure

By adding and removing tables and fields

By changing the characteristics of existing fields

By creating and dropping indexes

The exact manner in which these changes are accomplished varies from one system to another

Most systems allow all of these changes to be made quickly and easily

Made using the SQL ALTER TABLE command


Structure Changes – Add and Change

ALTER TABLE CustomerADD CustType CHAR(1);

Adding new field

Changing field properties

ALTER TABLE CustomerCHANGE COLUMN CustomerName TO CHAR(50);


Figure 4.22: Add Field in Access


Figure 4.23: Change Field Characteristic in Access


Structure Changes - Delete

ALTER TABLE PartDELETE Warehouse;

Deleting field

Delete SQL Table

DROP TABLE SmallCust;


Figure 4.24: Delete Field in Access


Figure 4.25: Delete Table in Access


System Catalog

Information about database kept in system catalog

Maintained by DBMS

Example catalog has two tables

Systables – information about the tables known to SQL

Syscolumns – information about the columns or fields within these tables


System Catalog (con’t.)

Other possible tables

Sysindexes – information about the indexes that are defined on these tables

Sysviews – information about the views that have been created


Figure 4.26: Systables Table


Summary Views - used to give each user his or her own view

of the data in a database

View is defined in structured query language (SQL) by using a defining query

Indexes are often used to facilitate data retrieval from the database

Security is provided in SQL systems using the GRANT and REVOKE commands

Entity integrity is the property that states that no field that is part of the primary key can accept null values


Summary

Referential integrity - property stating that the value in any foreign key field must either be null or match an actual value in the primary key field of another table

Legal-values integrity is the property that states that the value entered in a field must be one of the legal values

The ALTER TABLE command allows you to add fields to a table, delete fields, or change the characteristics of fields


Summary

The DROP TABLE command lets you delete a table from a database

The system catalog is a feature of many relational DBMSs that stores information about the structure of a database

Concepts of Database Management, Fifth Edition Chapter 4: The Relational Model 3: Advanced Topics.

Documents

Transcript of Concepts of Database Management, Fifth Edition Chapter 4: The Relational Model 3: Advanced Topics.