Returns NULL if expr1 = expr2 is true, otherwise returns expr1.
mysql>
mysql> SELECT NULLIF(1,1);
+-------------+
| NULLIF(1,1) |
+-------------+
| NULL |
+-------------+
1 row in set (0.00 sec)
mysql> SELECT NULLIF(NULL,2);
+----------------+
| NULLIF(NULL,2) |
+----------------+
| NULL |
+----------------+
1 row in set (0.00 sec)