Database Systems:

Design, Implementation, and

Management
Tenth Edition

Chapter 7
Introduction to Structured Query
Language (SQL)

Objectives
In this chapter, students will learn:
The basic commands and functions of SQL
How to use SQL for data administration (to
create tables and indexes)
How to use SQL for data manipulation (to add,
modify, delete, and retrieve data)
How to use SQL to query a database for useful
information
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Introduction to SQL
SQL functions fit into two broad categories:
Data definition language
Data manipulation language

Basic command set has vocabulary of fewer

than 100 words
American National Standards Institute (ANSI)
prescribes a standard SQL
Several SQL dialects exist
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Data Definition Commands

The database model
In this chapter, a simple database with these
tables is used to illustrate commands:

CUSTOMER
INVOICE
LINE
PRODUCT
VENDOR

Focus on PRODUCT and VENDOR tables

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Creating the Database

Two tasks must be completed:
Create database structure
Create tables that will hold end-user data

First task:
RDBMS creates physical files that will hold
database
Differs substantially from one RDBMS to another

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Creating the Database (contd.)

Authentication
DBMS verifies that only registered users are
able to access database
Log on to RDBMS using user ID and password
created by database administrator

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The Database Schema

Schema
Group of database objects that are related to
each other

CREATE SCHEMA AUTHORIZATION {creator};

Command is seldom used directly

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Data Types
Data type selection is usually dictated by nature
of data and by intended use
Supported data types:

Number(L,D), Integer, Smallint, Decimal(L,D)

Char(L), Varchar(L), Varchar2(L)
Date, Time, Timestamp
Real, Double, Float
Interval day to hour
Many other types

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Creating Table Structures

Use one line per column (attribute) definition
Use spaces to line up attribute characteristics
and constraints
Table and attribute names are capitalized
NOT NULL specification
UNIQUE specification

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Creating Table Structures (contd.)

Primary key attributes contain both a NOT
NULL and a UNIQUE specification
RDBMS will automatically enforce referential
integrity for foreign keys
Command sequence ends with semicolon

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SQL Constraints
NOT NULL constraint
Ensures that column does not accept nulls

UNIQUE constraint
Ensures that all values in column are unique

DEFAULT constraint
Assigns value to attribute when a new row is
added to table

CHECK constraint
Validates data when attribute value is entered
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SQL Indexes
When primary key is declared, DBMS
automatically creates unique index
Often need additional indexes
Using CREATE INDEX command, SQL indexes
can be created on basis of any selected
attribute
Composite index
Index based on two or more attributes
Often used to prevent data duplication
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Data Manipulation Commands

INSERT
SELECT
COMMIT
UPDATE
ROLLBACK
DELETE

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Adding Table Rows

INSERT
Used to enter data into table
Syntax:
INSERT INTO columnname
VALUES (value1, value2, , valueN);

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Adding Table Rows (contd.)

When entering values, notice that:
Row contents are entered between parentheses
Character and date values are entered between
apostrophes
Numerical entries are not enclosed in
apostrophes
Attribute entries are separated by commas
A value is required for each column

Use NULL for unknown values

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Saving Table Changes

Changes made to table contents are not
physically saved on disk until:
Database is closed
Program is closed
COMMIT command is used

Syntax:
COMMIT [WORK];

Will permanently save any changes made to

any table in the database
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Listing Table Rows

SELECT
Used to list contents of table
Syntax:
SELECT columnlist
FROM tablename;

Columnlist represents one or more attributes,

separated by commas
Asterisk can be used as wildcard character to
list all attributes
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Updating Table Rows

UPDATE
Modify data in a table
Syntax:
UPDATE tablename
SET columnname = expression [, columnname =
expression]
[WHERE conditionlist];

If more than one attribute is to be updated in

row, separate corrections with commas
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Restoring Table Contents

ROLLBACK
Undoes changes since last COMMIT
Brings data back to prechange values

Syntax:
ROLLBACK;

COMMIT and ROLLBACK only work with

commands to add, modify, or delete table rows

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Deleting Table Rows

DELETE
Deletes a table row
Syntax:
DELETE FROM tablename
[WHERE conditionlist ];

WHERE condition is optional

If WHERE condition is not specified, all rows
from specified table will be deleted

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Inserting Table Rows with a

SELECT Subquery
INSERT
Inserts multiple rows from another table (source)
Uses SELECT subquery
Subquery: query embedded (or nested or inner)
inside another query
Subquery executed first
Syntax:
INSERT INTO tablename SELECT columnlist
FROM tablename;
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SELECT Queries
Fine-tune SELECT command by adding
restrictions to search criteria using:

Conditional restrictions
Arithmetic operators
Logical operators
Special operators

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Selecting Rows with

Conditional Restrictions
Select partial table contents by placing
restrictions on rows to be included in output
Add conditional restrictions to SELECT
statement, using WHERE clause

Syntax:
SELECT columnlist
FROM tablelist
[ WHERE conditionlist ] ;

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Selecting Rows with

Conditional Restrictions (contd.)
Using comparison operators on dates
Date procedures are often more softwarespecific than other SQL procedures

Using computed columns and column aliases

SQL accepts any valid expressions (or formulas)
in the computed columns
Alias
Alternate name given to a column or table in any
SQL statement
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Arithmetic Operators:
The Rule of Precedence

Perform operations within parentheses

Perform power operations
Perform multiplications and divisions
Perform additions and subtractions

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Logical Operators: AND, OR, and NOT

Searching data involves multiple conditions
Logical operators: AND, OR, and NOT
Can be combined
Parentheses enforce precedence order
Conditions in parentheses are always executed
first

Boolean algebra: mathematical field dedicated

to use of logical operators
NOT negates result of conditional expression
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Special Operators
BETWEEN: checks whether attribute value is
within a range
IS NULL: checks whether attribute value is null
LIKE: checks whether attribute value matches
given string pattern
IN: checks whether attribute value matches any
value within a value list
EXISTS: checks if subquery returns any rows
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Advanced Data Definition Commands

All changes in table structure are made by
using ALTER command
Three options:
ADD adds a column
MODIFY changes column characteristics
DROP deletes a column

Can also be used to:

Add table constraints
Remove table constraints
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Changing a Columns Data Type

ALTER can be used to change data type
Some RDBMSs do not permit changes to data
types unless column is empty

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Changing a Columns Data

Characteristics
Use ALTER to change data characteristics
Changes in columns characteristics are
permitted if changes do not alter the existing
data type

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Adding a Column
Dropping a Column
Use ALTER to add column
Do not include the NOT NULL clause for new
column

Use ALTER to drop column

Some RDBMSs impose restrictions on the
deletion of an attribute

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Advanced Data Updates

UPDATE command updates only data in
existing rows
If relationship between entries and existing
columns, can assign values to slots
Arithmetic operators are useful in data updates
In Oracle, ROLLBACK command undoes
changes made by last two UPDATE statements

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Copying Parts of Tables

SQL permits copying contents of selected table
columns
Data need not be reentered manually into newly
created table(s)

First create the table structure

Next add rows to new table using table rows
from another table

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Adding Primary and Foreign Key

Designations
When table is copied, integrity rules do not
copy
Primary and foreign keys are manually defined
on new table

User ALTER TABLE command

Syntax:
ALTER TABLE tablename ADD PRIMARY
KEY(fieldname);

For foreign key, use FOREIGN KEY in place of

PRIMARY KEY
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Deleting a Table from the Database

DROP
Deletes table from database
Syntax:
DROP TABLE tablename;

Can drop a table only if it is not the one side

of any relationship
Otherwise, RDBMS generates an error message
Foreign key integrity violation

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Additional SELECT Query Keywords

Logical operators work well in the query
environment
SQL provides useful functions that:
Count
Find minimum and maximum values
Calculate averages, etc.

SQL allows user to limit queries to:

Entries having no duplicates
Entries whose duplicates may be grouped
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Ordering a Listing
ORDER BY clause is useful when listing order
is important
Syntax:
SELECT columnlist
FROM tablelist
[WHERE conditionlist]
[ORDER BY columnlist [ASC | DESC]];

Ascending order by default

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Listing Unique Values

DISTINCT clause produces list of only values
that are different from one another
Example:
SELECT DISTINCT V_CODE
FROM PRODUCT;

Access places nulls at the top of the list

Oracle places it at the bottom
Placement of nulls does not affect list contents
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Aggregate Functions
COUNT function tallies number of non-null values
of an attribute
Takes one parameter: usually a column name

MAX and MIN find highest (lowest) value in a table

Compute MAX value in inner query
Compare to each value returned by the query

SUM computes total sum for any specified

attribute
AVG function format is similar to MIN and MAX
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Grouping Data
Frequency distributions created by GROUP BY
clause within SELECT statement
Syntax:
SELECT
FROM
[WHERE
[GROUP BY
[HAVING
[ORDER BY
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columnlist
tablelist
conditionlist]
columnlist]
conditionlist]
columnlist [ASC | DESC] ] ;
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Joining Database Tables

Joining tables is the most important distinction
between relational database and other DBs
Join is performed when data are retrieved from
more than one table at a time
Equality comparison between foreign key and
primary key of related tables

Join tables by listing tables in FROM clause of

SELECT statement
DBMS creates Cartesian product of every table
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Joining Tables with an Alias

Alias identifies the source table from which data
are taken
Alias can be used to identify source table
Any legal table name can be used as alias
Add alias after table name in FROM clause
FROM tablename alias

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Recursive Joins
Alias is especially useful when a table must be
joined to itself
Recursive query
Use aliases to differentiate the table from itself

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Summary
SQL commands can be divided into two overall
categories:
Data definition language commands
Data manipulation language commands

The ANSI standard data types are supported by

all RDBMS vendors in different ways
Basic data definition commands allow you to
create tables and indexes
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Summary (contd.)
DML commands allow you to add, modify, and
delete rows from tables
The basic DML commands:
SELECT, INSERT, UPDATE, DELETE,
COMMIT, and ROLLBACK

SELECT statement is main data retrieval

command in SQL

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Summary (contd.)
WHERE clause can be used with SELECT,
UPDATE, and DELETE statements
Aggregate functions
Special functions that perform arithmetic
computations over a set of rows

ORDER BY clause
Used to sort output of SELECT statement
Can sort by one or more columns
Ascending or descending order
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Summary (contd.)
Join output of multiple tables with SELECT
statement
Join performed every time you specify two or
more tables in FROM clause
If no join condition is specified, DBMX performs
Cartesian product

Natural join uses join condition to match only

rows with equal values in specified columns

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