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
