Différence entre deux horodatages en secondes dans MySQL ?

mysql> SELECT TIMESTAMPDIFF(SECOND, '2018-10-17 11:51:55', '2018-10-17 11:51:58');

+---------------------------------------------------------------------+
| TIMESTAMPDIFF(SECOND, '2018-10-17 11:51:55', '2018-10-17 11:51:58') |
+---------------------------------------------------------------------+
|                                                                   3 |
+---------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql>  SELECT UNIX_TIMESTAMP('2018-10-17 11:57:50') - UNIX_TIMESTAMP('2018-10-17 11:57:45') as DifferenceInSeconds;

+---------------------+
| DifferenceInSeconds |
+---------------------+
|                   5 |
+---------------------+
1 row in set (0.00 sec)