SQL

SubQuery

1ICT Level V

COT - Jaffna

S.Sakthybaalan

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What Is a Subquery?

A subquery is a SELECT statement embedded in a

clause of another SQL statement (SELECT, INSERT,

UPDATE, DELETE).

SELECT . . .

FROM . . .

WHERE . . .(SELECT . . .

FROM . . .

WHERE . . .)

Main

Query

Subquery

SQL subquery is usually added in the WHERE Clause

of the SQL statement.

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The basic concept is to pass a single value

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When reading or writing SQL subqueries, you should start from

the bottom upwards, working out which data is to be passed to the next query up.

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SQL Subquery Subquery or Inner query or Nested query.

An alternate way of returning data from multiple tables.

Subqueries can be used with the following SQL statements

along with the comparision operators like =, <, >, >=, <= etc.

Subqueries can also be used inside the WHERE or HAVING

clause.

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Sub-query Syntax

• The subquery (inner query) executes once

before the main query.

• The result of the subquery is used by the main

query (outer query).

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Using a Subquery to Solve a

Problem To solve this problem, you need

two queries: one to find what Abel earns, and a second query to find who earns more than that amount.

You can solve this problem by combining the two queries, placing one query inside the other query.

The inner query or the subqueryreturns a value that is used by the outer query or the main query.

Using a subquery is equivalent to performing two sequential queries and using the result of the first query as the search value in the second query.

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Example :

In the slide, the inner query determines the salary of employee Abel.

The outer query takes the result of the inner query and uses this result to display all the employees who earn more than this amount.

Execute the subquery (inner query) on its own first to show the value that the subquery returns.

Then execute the outer query using the result returned by the inner query. Finally, execute the entire query (containing the subquery), and show that the result is the same.

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Types of SubQueries

SubQueries

Single-row subqueries Multiple-row subqueries

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Types of Subqueries

Single-row subqueries:Queries that return only one

row from the inner SELECT

statement

Multiple-row subqueries:Queries that return more than

one row from the inner SELECT

statement

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Single-Row Subqueries

Return only one row

Use single-row comparison operators

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Executing Single-Row Subqueries

A SELECT statement can be considered as a query block.

The example on the slide displays employees whose job ID is the same as that of employee 141 and whose salary is greater than that of employee 143.

The example consists of three query blocks: the outer query and two inner queries.

The inner query blocks are executed first, producing the query results ST_CLERK and 2600, respectively.

The outer query block is then processed and uses the values returned by the inner queries to complete its search conditions.

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Problems with Subqueries

A common problem with subqueries is no rows being returned by the inner query.

There is no employee named Haas. So the subquery returns no rows.

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Multiple-Row Subqueries Subqueries that return more

than one row are calledmultiple-row subqueries.

You use a multiple-rowoperator, instead of a single-row operator, with amultiple-row subquery.

The multiple-row operatorexpects one or more values.

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Example :

The NOT operator can be used with IN, ANY, and ALL operators.

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Exercise 1 :

Track

cID Num Title Time aID

1 1 Violent 239 11 2 Every Girl 410 11 3 Breather 217 11 4 Part of Me 279 12 1 Star 362 12 2 Teaboy 417 2

CD

cID Title Price

1 Mix 9.992 Compilation 12.99

Artist

aID Name

1 Stellar2 Cloudboy

• Find a list of all the CD titles.

• Find a list of the titles of tracks that are more than 300 seconds long.

• Find a list of the names of those artists who have a track on the CD

with the title “Compilation”.

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LOCATION

Location_ID Regional_Group

122 NEW YORK

123 DALLAS

124 CHICAGO

167 BOSTON

DEPARTMENT

Department_ID Name Location_ID

10 ACCOUNTING 122

20 RESEARCH 124

30 SALES 123

40 OPERATIONS 167

JOB

Job_ID Function

667 CLERK

668 STAFF

669 ANALYST

670 SALESPERSON

671 MANAGER

672 PRESIDENT

EMPLOYEE

EMPLOYEE

_IDLAST_NAME FIRST_NAME

MIDDLE_

NAMEJOB_ID

MANAGER

_IDHIREDATE SALARY COMM

DEPARTME

NT_ID

7369 SMITH JOHN Q 667 7902 17-DEC-84 800 NULL 20

7499 ALLEN KEVIN J 670 7698 20-FEB-85 1600 300 30

7505 DOYLE JEAN K 671 7839 04-APR-85 2850 NULL 30

7506 DENNIS LYNN S 671 7839 15-MAY-85 2750 NULL 30

7507 BAKER LESLIE D 671 7839 10-JUN-85 2200 NULL 40

7521 WARK CYNTHIA D 670 7698 22-FEB-85 1250 500 30

Exercise 2 :

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1. Display the employee who got the maximum salary.

SELECT *

FROM employee

WHERE salary =(SELECT MAX(salary) FROM employee)

2. Find out no.of employees working in “Sales” department.

SELECT *

FROM employee

WHERE department_id =(SELECT department_id

FROM department

WHERE name=’SALES’

GROUP BY department_id)

Sub-Queries

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3. Delete the employees who are working in accounting

department.

DELETE FROM employee

WHERE department_id =(SELECT department_id

FROM department

WHERE name=’ACCOUNTING’)

4. Display the second highest salary drawing employee details.

SELECT *

FROM employee

WHERE salary =( SELECT MAX(salary)

FROM employee

WHERE salary <(SELECT MAX(salary)

FROM employee))

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Any Questions ?

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