Syntax:
CASE case_value
    WHEN when_value THEN statement_list
    [WHEN when_value THEN statement_list] ...
    [ELSE statement_list]
END CASE

Or:

CASE
    WHEN search_condition THEN statement_list
    [WHEN search_condition THEN statement_list] ...
    [ELSE statement_list]
END CASE

The CASE statement for stored programs implements a complex conditional
construct. If a search_condition evaluates to true, the corresponding
SQL statement list is executed. If no search condition matches, the
statement list in the ELSE clause is executed. Each statement_list
consists of one or more statements.

If no when_value or search_condition matches the value tested and the
CASE statement contains no ELSE clause, a Case not found for CASE
statement error results.

Each statement_list consists of one or more statements; an empty
statement_list is not permitted. To handle situations where no value is
matched by any WHEN clause, use an ELSE containing an empty BEGIN ...
END block, as shown in this example: DELIMITER | CREATE PROCEDURE p()
BEGIN DECLARE v INT DEFAULT 1; CASE v WHEN 2 THEN SELECT v; WHEN 3 THEN
SELECT 0; ELSE BEGIN END; END CASE; END; | (The indentation used here
in the ELSE clause is for purposes of clarity only, and is not
otherwise significant.)

*Note*: The syntax of the CASE statement used inside stored programs
differs slightly from that of the SQL CASE expression described in
http://dev.mysql.com/doc/refman/5.5/en/control-flow-functions.html. The
CASE statement cannot have an ELSE NULL clause, and it is terminated
with END CASE instead of END.

URL: http://dev.mysql.com/doc/refman/5.5/en/case-statement.html

