How to make a unique datetime without seconds?
Store CONCAT(LEFT(NOW(), 16), ':00') into the PRIMARY KEY that is a DATETIME.

mysql&gt; SELECT NOW(), CONCAT(LEFT(NOW(), 16), ':00');
+---------------------+--------------------------------+
| NOW()               | CONCAT(LEFT(NOW(), 16), ':00') |
+---------------------+--------------------------------+
| 2015-12-11 21:37:38 | 2015-12-11 21:37:00            |
+---------------------+--------------------------------+