A subquery is a SELECT statement embedded within another SQL statement. It allows queries to retrieve data from multiple tables or queries. There are two types of subqueries: single-row and multiple-row. Single-row subqueries return only one row of data and use single-row comparison operators like =. Multiple-row subqueries return more than one row of data and use operators like IN, ANY, ALL that can handle multiple values. Subqueries are useful for solving problems that require performing multiple related queries by nesting one query within another.

1. SQL
SubQuery
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ICT 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 subquery
returns 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 called
multiple-row subqueries.
 You use a multiple-row
operator, instead of a single-
row operator, with a
multiple-row subquery.
 The multiple-row operator
expects 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 1
1 2 Every Girl 410 1
1 3 Breather 217 1
1 4 Part of Me 279 1
2 1 Star 362 1
2 2 Teaboy 417 2
CD
cID Title Price
1 Mix 9.99
2 Compilation 12.99
Artist
aID Name
1 Stellar
2 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
_ID
LAST_NAME FIRST_NAME
MIDDLE_
NAME
JOB_ID
MANAGER
_ID
HIREDATE 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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