SQL Pocket Guide, 3rd Edition

CASE Expressions: Simple

Simple CASE expressions correlate a list of values to a list of alternatives. For example:

SELECT u.name,
   CASE u.open_to_public
      WHEN 'y' THEN 'Welcome!'
      WHEN 'n' THEN 'Go Away!'
      ELSE 'Bad code!'
   END AS column_alias
FROM upfall u;

Simple CASE expressions are useful when you can directly link an input value to a WHEN clause by means of an equality condition. If no WHEN clause is a match, and no ELSE is specified, the expression returns null.