   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.
