CASE (expression)
A CASE expression returns a value. CASE expression has two formats:
•Simple When/Then. This format compares a value expression to a set of value expressions to determine a result. The value expressions are evaluated in their order listed. If a value expression evaluates to TRUE, CASE returns the value expression for the THEN clause.
•Searched When/Then. This format evaluates a set of Boolean expressions to determine a result. The Boolean expressions are evaluated in their order listed. If a Boolean expression evaluates to TRUE, CASE returns the expression for the THEN clause.
Both formats support an optional ELSE argument. If no ELSE clause is used, then ELSE NULL is implied.
Syntax
Simple When/Then:
CASE case_value_expression
WHEN when_expression THEN then_expression [...]
[ ELSE else_expression ]
END
Searched When/Then:
CASE
WHEN search_expression THEN then_expression [...]
[ ELSE else_expression ]
END
Arguments
case_value_expression ::= the expression evaluated by the simple When/Then CASE format.
when_expression ::= The expression to which case_value_expression is compared. The data types of case_value_expression and each when_expression must be the same or must be an implicit conversion.
then_expression ::= the expression returned when case_value_expression equals when_expression evaluates to TRUE.
else_expression ::= the expression returned if no comparison operation evaluates to TRUE. If this argument is omitted and no comparison operation evaluates to TRUE, CASE returns NULL.
search_expression ::= the Boolean expression evaluated by the searched CASE format. Search_expression may be any valid Boolean expression.
Remarks
A CASE expression must be used within a SELECT statement. The SELECT statement may be within a stored procedure or within a view.
Examples
The following statement uses the simple When/Then format to report the prerequisites for the art courses listed in the Course table.
SELECT name 'Course ID', description 'Course Title',
CASE name
WHEN 'Art 101' THEN 'None'
WHEN 'Art 102' THEN 'Art 101 or instructor approval'
WHEN 'Art 203' THEN 'Art 102'
WHEN 'Art 204' THEN 'Art 203'
WHEN 'Art 305' THEN 'Art 101'
WHEN 'Art 406' THEN 'None'
WHEN 'Art 407' THEN 'Art 305'
END
AS 'Prerequisites' FROM Course WHERE Dept_Name = 'Art' ORDER BY name
The query returns the following:
Course ID | Course Title | Prerequisites |
Art 101 | Drawing I | None |
Art 102 | Drawing II | Art 101 or instructor approval |
Art 203 | Drawing III | Art 102 |
Art 204 | Drawing IV | Art 203 |
Art 305 | Sculpture | Art 101 |
Art 406 | Modern Art | None |
Art 407 | Baroque Art | Art 305 |
============
The previous statement can be changed to include an ELSE clause:
SELECT name 'Course ID', description 'Course Title',
CASE name
WHEN 'Art 101' THEN 'None'
WHEN 'Art 102' THEN 'Art 101 or instructor approval'
WHEN 'Art 203' THEN 'Art 102'
WHEN 'Art 204' THEN 'Art 203'
WHEN 'Art 305' THEN 'Art 101'
ELSE 'Curriculum plan for Art History majors'
END
AS 'Prerequisites' FROM Course WHERE Dept_Name = 'Art' ORDER BY name
The query now returns the following:
Course ID | Course Title | Prerequisites |
Art 101 | Drawing I | None |
Art 102 | Drawing II | Art 101 or instructor approval |
Art 203 | Drawing III | Art 102 |
Art 204 | Drawing IV | Art 203 |
Art 305 | Sculpture | Art 101 |
Art 406 | Modern Art | Curriculum plan for Art History majors |
Art 407 | Baroque Art | Curriculum plan for Art History majors |
============
The following statement uses the searched When/Then format to report the funding program for which a person may be eligible.
SELECT last_name, first_name,
CASE
WHEN scholarship = 1 THEN 'Scholastic'
WHEN citizenship <> 'United States' THEN 'Foreign Study'
WHEN (date_of_birth >= '1960-01-01' AND date_of_birth <= '1970-01-01') THEN 'AJ-44 Funds'
ELSE 'NONE'
END
AS 'Funding Program' FROM Person ORDER BY last_name
Here is a partial listing of what the query returns:
Last_Name | First_Name | Funding Program |
Abad | Alicia | NONE |
Abaecherli | David | Foreign Study |
Abebe | Marta | Foreign Study |
Abel | James | AJ-44 Funds |
Abgoon | Bahram | Foreign Study |
Abken | Richard | NONE |
Abu | Austin | Foreign Study |
Abuali | Ibrahim | AJ-44 Funds |
Acabbo | Joseph | NONE |
Acar | Dennis | Foreign Study |
============
The following example shows how a CASE expression may be used within a stored procedure.
CREATE PROCEDURE pcasetest () RETURNS (d1 CHAR(10), d2 CHAR(10));
BEGIN
SELECT c1, CASE WHEN c1 = 1 THEN c4
WHEN c1 = 2 THEN c5
ELSE
CASE WHEN c2 = 100.22 THEN c4
WHEN c2 = 101.22 THEN c5 END END
FROM tcasetest;
END
CALL pcasetest
============
The following example shows how a CASE expression may be used within a view.
CREATE VIEW vcasetest (vc1, vc2) AS
SELECT c1, CASE WHEN c1 = 1 THEN c4
WHEN c1 = 2 THEN c5
ELSE
CASE WHEN c2 = 100.22 THEN c4
WHEN c2 = 101.22 THEN c5 END END
FROM TCASEWHEN
SELECT * FROM vcasetest
See Also