NULL-safe equal() performs an equality comparison like the = operator.
NULL-safe equal() returns 1 rather than NULL if both operands are NULL.
NULL-safe equal() returns 0 rather than NULL if one operand is NULL.
mysql>
mysql> SELECT 1 <=> 1, NULL <=> NULL, 1 <=> NULL;
+---------+---------------+------------+
| 1 <=> 1 | NULL <=> NULL | 1 <=> NULL |
+---------+---------------+------------+
| 1 | 1 | 0 |
+---------+---------------+------------+
1 row in set (0.00 sec)
mysql>
mysql> SELECT 1 = 1, NULL = NULL, 1 = NULL;
+-------+-------------+----------+
| 1 = 1 | NULL = NULL | 1 = NULL |
+-------+-------------+----------+
| 1 | NULL | NULL |
+-------+-------------+----------+
1 row in set (0.00 sec)
mysql>