Significantly both ISNULL() function and IS NULL operator do not have any difference and shares some common behaviors’. The only difference which we can see is in their syntax. ISNULL() function would have the expression as its argument whereas IS NULL comparison operator is having the expression to its left. Otherwise, both returns 1 if an expression is NULL and 0 if the expression is not NULL. Following example will demonstrate the above concept −
mysql> Select 1 IS NULL; +-----------+ | 1 IS NULL | +-----------+ | 0 | +-----------+ 1 row in set (0.00 sec) mysql> Select ISNULL(1); +-----------+ | ISNULL(1) | +-----------+ | 0 | +-----------+ 1 row in set (0.00 sec) mysql> Select ISNULL(1/0); +-------------+ | ISNULL(1/0) | +-------------+ | 1 | +-------------+ 1 row in set (0.00 sec) mysql> Select 1/0 IS NULL; +-------------+ | 1/0 IS NULL | +-------------+ | 1 | +-------------+ 1 row in set (0.00 sec) mysql> Select * from Employee WHERE Salary IS NULL; +----+-------+--------+ | ID | Name | Salary | +----+-------+--------+ | 7 | Aryan | NULL | | 8 | Vinay | NULL | +----+-------+--------+ 2 rows in set (0.00 sec) mysql> Select * from Employee WHERE ISNULL(Salary); +----+-------+--------+ | ID | Name | Salary | +----+-------+--------+ | 7 | Aryan | NULL | | 8 | Vinay | NULL | +----+-------+--------+ 2 rows in set (0.00 sec)