TIME_TO_SEC()

TIME_TO_SEC(time)

Devuelve el argumento time convertido en segundos:

mysql> SELECT TIME_TO_SEC('22:23:00');
+-------------------------+
| TIME_TO_SEC('22:23:00') |
+-------------------------+
|                   80580 |
+-------------------------+
1 row in set (0.00 sec)

mysql> SELECT TIME_TO_SEC('00:39:38');
+-------------------------+
| TIME_TO_SEC('00:39:38') |
+-------------------------+
|                    2378 |
+-------------------------+
1 row in set (0.00 sec)