IS NOT 'boolean_value': Tests a value against a boolean value, where boolean_value can be TRUE, FALSE, or UNKNOWN : IS « Comparison Functions Operators « MySQL Tutorial

1. MySQL Tutorial
2. Comparison Functions Operators
3. IS

mysql>
mysql> SELECT 1 IS NOT UNKNOWN, 0 IS NOT UNKNOWN, NULL IS NOT UNKNOWN;
+------------------+------------------+---------------------+
| 1 IS NOT UNKNOWN | 0 IS NOT UNKNOWN | NULL IS NOT UNKNOWN |
+------------------+------------------+---------------------+
|                1 |                1 |                   0 |
+------------------+------------------+---------------------+
1 row in set (0.00 sec)

mysql>