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.

*Note*: There is also a CASE expression, which differs from the CASE
statement described here. See
http://dev.mysql.com/doc/refman/5.6/en/control-flow-functions.html. The
CASE statement cannot have an ELSE NULL clause, and it is terminated
with END CASE instead of END.

For the first syntax, case_value is an expression. This value is
compared to the when_value expression in each WHEN clause until one of
them is equal. When an equal when_value is found, the corresponding
THEN clause statement_list executes. If no when_value is equal, the
ELSE clause statement_list executes, if there is one.

This syntax cannot be used to test for equality with NULL because NULL
= NULL is false. See
http://dev.mysql.com/doc/refman/5.6/en/working-with-null.html.

For the second syntax, each WHEN clause search_condition expression is
evaluated until one is true, at which point its corresponding THEN
clause statement_list executes. If no search_condition is equal, the
ELSE clause statement_list executes, if there is one.

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 SQL 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. (The indentation used here in the ELSE clause is for purposes
of clarity only, and is not otherwise significant.)

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;
  |

URL: http://dev.mysql.com/doc/refman/5.6/en/case.html

