A Guide to SQL, Seventh Edition. Objectives Understand, create, and drop views Recognize the benefits of using views Grant and revoke user’s database.

A Guide to SQL, Seventh Edition

Objectives

Understand, create, and drop views

Recognize the benefits of using views

Grant and revoke user’s database privileges

Understand the purpose, advantages, and disadvantages of using an index


Objectives

Create, use, and drop an index

Understand and obtain information from the system catalog

Use integrity constraints to control data entry


Views

A view is an application program’s or individual user’s picture of the database

Base tables are the existing, permanent tables in a relational database

A view is a derived table because data in it is retrieved from the base table


Views

To user, a view appears as an actual table, but it is not

A view usually includes less information than the full database

Its use represents a great simplification

Provides a measure of security by omitting sensitive information unavailable to user


Defining Views

A view is defined by creating a defining query

Indicates rows and columns to include

Use CREATE VIEW command

CREATE VIEW, followed by the name of the view, AS, and then a query


Defining Views

MySQL does not support views at this time

Access users would create the defining query and save the query with view name

Data shown in the view does not exist in this form, nor will it ever

Not a temporary table




Queries Involving Views

If a query involves a view, SQL changes the query to select data from the table in the database that created the view

DBMS does not execute the query in the form

Entered query is merged with original query that defined the view to create the final query that is executed


The query executed by SQL


Renaming Columns In a View

Column names can be assigned in new view

Include new column names in parentheses, following the name of the view

Output will display new column names



View Joining Two Tables

Defining query of a view can be any valid SQL query A view can join two or more tables



A View with Statistics A view can involve statistics


Benefits of Views

Views provide data independence

Views can often be used even after database structure changes

Different users can view same data differently

A view can contain only those columns required by a given user


Row-and-Column Subsets

When attempting to add a row to a row-and-column subset view, the DBMS must determine how to enter data

Provided every column not included in a view can accept nulls, use INSERT

Data may be rejected on some attempts when problems arise


Row-and-Column Subsets Updates or deletes are not a problem in this view Not every row-and-column subsets are updatable


Joins Views that involve joins of base tables can cause

problems at update



Updatable Views

Updatable view is when a view is derived by joining two tables on primary key of each table

Views involving joins by matching the primary key of one table with a column that is not the primary key are not updatable

More severe problems are encountered if neither of the join columns is a primary key


Statistics

Most difficult views to update are those that involve statistics

You can not add rows to a view that includes calculations


Dropping a View

Remove a view that is no longer needed with DROP VIEW command

The DROP VIEW command removes only the view definition

Table and data remain unchanged


Security

Security is the prevention of unauthorized access to a database

Some may be able to retrieve and update anything in a database

Others may be able to retrieve data but not change data

Others may be able to access only a portion of data


Access Using the GRANT Command

Main mechanism for providing access to a database is the GRANT command

Database administrator can grant different types of privileges to users and revoke them later

Privileges include rights to select, insert, update, index, and delete table data


Using the REVOKE Command

Database administrator uses the REVOKE command to remove privileges from users

Format is similar to GRANT command


Indexes

An index speeds up the searching of tables

Similar to an index in a book

Indexes are more complicated than that shown in the figures



Indexes

The DBMS system manages indexes

User determines the columns on which to build indexes

Disadvantages

Index occupies disk space

DBMS must update the index as data is entered



Dropping an Index

The command to drop or delete an index is DROP INDEX

DROP INDEX followed by the name of the index to drop

Permanently deletes index


Unique Indexes

To ensure uniqueness of non-primary key data, you can create a unique index

Command is CREATE UNIQUE INDEX

A unique index will reject any update that would cause a duplicate value in the specified column


System Catalog

Information about tables in the database is kept in the system catalog or data dictionary

Describes types of items kept in the catalog

Also describes the way in which you can query it to access information about the database structure


System Catalog

The DBMS automatically maintains system catalog

SYSTABLES, information about the tables known to SQL

SYSCOLUMNS, information about the columns within those tables

SYSVIEWS, information about the views that have been used






Integrity Rules in SQL

An integrity constraint is a rule for the data in the database

Examples in Premier Products

• A sales rep’s number must be unique

• The sales rep number for a customer must match an exiting sales rep number

• Item classes for parts must be AP, HW, or SG


Integrity Support

Integrity support is the process of specifying integrity constraints for the database

Clauses to support three types of integrity constraints that can be specified within a CREATE TABLE or ALTER TABLE command


Adding and Changing Integrity Constraints

An ALTER TABLE command is followed by ADD to indicate the addition of the constraint

To change an integrity constraint, simply enter a new constraint

New constraint will immediately replace the original


Types of Constraints

Primary keys Use ADD PRIMARY KEY clause on ALTER TABLE

command to add after creating a table Foreign keys

A column in one table whose value matches the primary key in another

Legal values The CHECK clause ensures only legal values are

allowed in a given column




Parent and Child

When specifying a foreign key, the table containing the foreign key is the child

Table referenced by the foreign key is the parent


CHECK Clause

The CHECK clause of the ALTER TABLE command is used to ensure only legal values satisfying a particular condition are allowed in a given column

Check (Class in (‘AP’, ‘HW’, ‘SG’) )

or

Check (Class = ‘AP’ OR CLASS = ‘HW’ OR CLASS = ‘SG’)



Summary

The purpose, creation, use, and benefit of views

Examined the features related to security

The purpose, advantages, and disadvantages of using indexes

Add and drop indexes

System catalog information

Integrity constraints


A Guide to SQL, Seventh Edition. Objectives Understand, create, and drop views Recognize the benefits of using views Grant and revoke user’s database.

Documents

edition slide

new view

view dbms

view output

view command

view column names

column subset view

valid sql query