Syntax
NULLIF(expr1, expr2)
Purpose
If expr1 = expr2, the return value is NULL. Otherwise, the return value is expr1. It is the same as CASE WHEN expr1 = expr2 THEN NULL ELSE expr1 END. If the arguments are not equal, the value of the two calculations is expr1.
Examples
obclient> SELECT NULLIF('ABC', 123), NULLIF('123',123), NULLIF(NULL, 'abc');
+--------------------+-------------------+---------------------+
| NULLIF('ABC', 123) | NULLIF('123',123) | NULLIF(NULL, 'abc') |
+--------------------+-------------------+---------------------+
| ABC | NULL | NULL |
+--------------------+-------------------+---------------------+
1 row in set, 1 warning