Syntax
ISNULL(expr)
Purpose
If expr is NULL, ISNULL() returns 1. Otherwise, it returns 0.
You can use ISNULL() instead of the equal sign (=) to determine whether a value is NULL. This is because if the equal sign is used for the comparison between any value and a NULL value, the return value is always NULL. The ISNULL() function shares some common features with the IS NULL comparison operator. For more information about comparison operators, see Comparison operators.
Examples
obclient> SELECT ISNULL(null), ISNULL('test'), ISNULL(123.456), ISNULL('10:00');
+--------------+----------------+-----------------+-----------------+
| ISNULL(null) | ISNULL('test') | ISNULL(123.456) | ISNULL('10:00') |
+--------------+----------------+-----------------+-----------------+
| 1 | 0 | 0 | 0 |
+--------------+----------------+-----------------+-----------------+
1 row in set
obclient> SELECT ISNULL(null+1);
+----------------+
| ISNULL(null+1) |
+----------------+
| 1 |
+----------------+
1 row in set