The range of the return value is 0 to 23.
The range of TIME values could be much larger.
mysql> mysql> SELECT HOUR('10:05:03'); +------------------+ | HOUR('10:05:03') | +------------------+ | 10 | +------------------+ 1 row in set (0.00 sec) mysql> mysql> SELECT HOUR('272:59:59'); +-------------------+ | HOUR('272:59:59') | +-------------------+ | 272 | +-------------------+ 1 row in set (0.00 sec)