This document discusses subqueries in SQL. It defines subqueries as SELECT statements embedded within other SQL statements and explains that they allow queries to be based on unknown criteria. The document outlines guidelines for using subqueries and distinguishes between single-row subqueries, which return one row, and multiple-row subqueries, which return multiple rows. It provides examples of how to write queries using both types of subqueries.

1. Chapter 9
Subqueries

2. Objectives
Describe the types of problems that
subqueries can solve
Define subqueries
List the types of subqueries
Write single-row and multiple-row
subqueries

3. Using a Subquery
to Solve a Problem
Who has a GPA higher than Owen’s?
Which students have a GPA higher than
Owen’s GPA?
Main Query
What is Owen’s GPA?
Subquery
?

4. Subqueries
The subquery (inner query) executes once
before the main query.
The result of the subquery is used by the
main query (outer query).
SELECT select_list
FROM table
WHERE expr operator
(SELECT select_list
FROM table);

5. Using a Subquery
“Who has a GPA higher than Owen’s?”
LastName
GPA
Lee 3.82
Tham 3.89
SELECT LastName, GPA
from Student
where GPA >
(Select GPA from Student
Where LastName = 'Owen');
3.34

6. Guidelines for Using Subqueries
Enclose subqueries in parentheses.
Place subqueries on the right side of the
comparison operator.
Do not add an ORDER BY clause to a
subquery.
Use single-row operators with single-row
subqueries.

7. Types of Subqueries
Single-row subquery
Multiple-row subquery
Main query
Subquery
returns
DIC
DIC
DCS
Main query
Subquery
returns

8. Single-Row Subqueries
Return only one row
Use single-row comparison operators
Operator
=
>
>=
<
<=
<>
Meaning
Equal to
Greater than
Greater than or equal to
Less than
Less than or equal to
Not equal to

9. Retrieving data using Single-Row Subqueries
SELECT LastName, CourseID
from Student
where CourseID = Select CourseID from Student
Where LastName = 'Lewis');
Who study the same course as Lewis?
OUTPUT:
LastName CourseID
Lewis DICT
Nicosia DICT
Maser DICT
DICT

10. Retrieving data using Single-Row Subqueries
SELECT LastName,GroupLeader
from Student
where GroupLeader = (Select GroupLeader from Student
Where LastName = 'Williams');
S010
Who has the same group leader as Williams?
OUTPUT:
LastName GroupLeader
Mikulski S010
Faga S010
Williams S010

11. Retrieving data using Single-Row Subqueries
SELECT LastName,DateEnrolled
from Student
where DateEnrolled < (Select DateEnrolled from Student
Where StudID = 'S009');
Who have been enrolled earlier than student S009?
01-Feb-02
OUTPUT: LastName DateEnrolled
Kebel 23-Jun-01
Lee 05-Jan-02
Lewis 03-Mar-00
Law 01-Apr-01
Faga 25-Jun-01
Owen 17-Sep-01
Ng 01-Apr-01
Roche 30-Mar-00
Jann 01-Apr-01

12. Retrieving data using Single-Row Subqueries
with GROUP functions
Display all the students that earn the minimum GPA
SELECT LastName,GPA
from Student
where GPA = (Select min(GPA)
from student);
1.88
OUTPUT:
LastName GPA
Ng 1.88
Roche 1.88

13. Multiple-Row Subqueries
Return more than one row
Use the IN multiple-row comparison
operator to compare an expression to any
member in the list that a subquery returns

14. Using Group Functions
in a Multiple-Row Subquery
Display all students who earn the same GPA as the
minimum GPA for each course
1.88,1.89,2.22,3
SELECT LastName,GPA, CourseID
from Student
where GPA IN
(select min(GPA)
from student
Group By CourseID);
OUTPUT: LastName GPA CourseID
Mikulski 1.89 DCS
Faga 2.22 DIC
Ng 1.88 DIT
Maser 3 DICT
Roche 1.88 DIT

15. Using Group Functions
in a Multiple-Row Subquery
Display all students who enrolled the same course as Law and Lewis.
DICT, DIT
SELECT LastName, CourseID
from Student
where CourseID IN
(select CourseID
from Student
where LastName IN ('Lewis','Law'))
order by CourseID;
OUTPUT:
LastName CourseID
Maser DICT
Nicosia DICT
Lewis DICT
Roche DIT
Ng DIT
Law DIT
Lee DIT

16. Summary
A subquery is a SELECT statement that is
embedded in a clause of another SQL
statement. Subqueries are useful when a query
is based on unknown criteria.
Subqueries have the following characteristics:
 Can pass one row of data to a main statement that
contains a single-row operator, such as =, <>, >, >=,
<, or <=
 Can pass multiple rows of data to a main statement
that contains a multiple-row operator, such as IN
 Can contain group functions