Syntax:
CASE value WHEN [compare_value] THEN result [WHEN [compare_value] THEN
result ...] [ELSE result] END

CASE WHEN [condition] THEN result [WHEN [condition] THEN result ...]
[ELSE result] END

The first version returns the result where value=compare_value. The
second version returns the result for the first condition that is true.
If there was no matching result value, the result after ELSE is
returned, or NULL if there is no ELSE part.

URL: http://dev.mysql.com/doc/refman/5.5/en/control-flow-functions.html

