The SQL CASE expression is a generic conditional expression, similar to if/else statements in other programming languages:

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

An example:

SELECT * FROM test;

 a
---
 1
 2
 3


SELECT a,
       CASE WHEN a=1 THEN 'one'
            WHEN a=2 THEN 'two'
            ELSE 'other'
       END
    FROM test;

 a | case
---+-------
 1 | one
 2 | two
 3 | other

http://www.postgresql.org/docs/current/static/functions-conditional.html