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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
A Guide to SQL, Seventh Edition
Objectives

Create, use, and drop an index

Understand and obtain information from the
system catalog

Use integrity constraints to control data entry
A Guide to SQL, Seventh Edition
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
A Guide to SQL, Seventh Edition
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
A Guide to SQL, Seventh Edition
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
A Guide to SQL, Seventh Edition
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
A Guide to SQL, Seventh Edition
A Guide to SQL, Seventh Edition
A Guide to SQL, Seventh Edition
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
A Guide to SQL, Seventh Edition
The query executed by SQL
A Guide to SQL, Seventh Edition
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
A Guide to SQL, Seventh Edition
A Guide to SQL, Seventh Edition
View Joining Two Tables


Defining query of a view can be any valid SQL query
A view can join two or more tables
A Guide to SQL, Seventh Edition
A Guide to SQL, Seventh Edition
A View with Statistics

A view can involve statistics
A Guide to SQL, Seventh Edition
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
A Guide to SQL, Seventh Edition
Row-and-Column Subsets

When attempting to add a row to a row-andcolumn 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
A Guide to SQL, Seventh Edition
Row-and-Column Subsets


Updates or deletes are not a problem in this view
Not every row-and-column subsets are updatable
A Guide to SQL, Seventh Edition
Joins

Views that involve joins of base tables can cause
problems at update
A Guide to SQL, Seventh Edition
A Guide to SQL, Seventh Edition
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
A Guide to SQL, Seventh Edition
Statistics

Most difficult views to update are those that
involve statistics

You can not add rows to a view that includes
calculations
A Guide to SQL, Seventh Edition
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
A Guide to SQL, Seventh Edition
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
A Guide to SQL, Seventh Edition
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
A Guide to SQL, Seventh Edition
Using the REVOKE Command

Database administrator uses the REVOKE
command to remove privileges from users

Format is similar to GRANT command
A Guide to SQL, Seventh Edition
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
A Guide to SQL, Seventh Edition
A Guide to SQL, Seventh Edition
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
A Guide to SQL, Seventh Edition
A Guide to SQL, Seventh Edition
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
A Guide to SQL, Seventh Edition
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
A Guide to SQL, Seventh Edition
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
A Guide to SQL, Seventh Edition
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
A Guide to SQL, Seventh Edition
A Guide to SQL, Seventh Edition
A Guide to SQL, Seventh Edition
A Guide to SQL, Seventh Edition
A Guide to SQL, Seventh Edition
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
A Guide to SQL, Seventh Edition
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
A Guide to SQL, Seventh Edition
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
A Guide to SQL, Seventh Edition
Types of Constraints

Primary keys


Foreign keys


Use ADD PRIMARY KEY clause on ALTER TABLE
command to add after creating a table
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
A Guide to SQL, Seventh Edition
A Guide to SQL, Seventh Edition
A Guide to SQL, Seventh Edition
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
A Guide to SQL, Seventh Edition
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’)
A Guide to SQL, Seventh Edition
A Guide to SQL, Seventh Edition
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