Logical NOT.
Return 1 if the operand is 0.
Return 0 if the operand is non-zero.
mysql>
mysql> SELECT NOT 10;
+--------+
| NOT 10 |
+--------+
| 0 |
+--------+
1 row in set (0.00 sec)
mysql> SELECT NOT 0;
+-------+
| NOT 0 |
+-------+
| 1 |
+-------+
1 row in set (0.00 sec)