IS '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 TRUE, 0 IS FALSE, NULL IS UNKNOWN;
+-----------+------------+-----------------+
| 1 IS TRUE | 0 IS FALSE | NULL IS UNKNOWN |
+-----------+------------+-----------------+
|         1 |          1 |               1 |
+-----------+------------+-----------------+
1 row in set (0.00 sec)

mysql>