CASE expressions can be used in place of any value-expressions.
Note that the following example does not specify an ELSE clause. ELSE clauses are always optional in a CASE expression. If an ELSE clause is not specified and none of the WHEN conditions are TRUE, then a null is returned.
SELECT * FROM t WHERE x = CASE WHEN y=2 THEN 1 WHEN (z=3 AND y=5) THEN 2 END;