Syntax:
NULLIF(expr1,expr2)

Returns NULL if expr1 = expr2 is true, otherwise returns expr1. This is
the same as CASE WHEN expr1 = expr2 THEN NULL ELSE expr1 END.

The return value has the same type as the first argument.

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

