MySQL DATETIME インスタンスから合計秒数を取得するにはどうすればよいですか?

mysql> Select UNIX_TIMESTAMP('2017-05-15 04:05:30') AS 'NUMBER OF SECONDS';

+-------------------+
| NUMBER OF SECONDS |
+-------------------+
|        1494801330 |
+-------------------+
1 row in set (0.00 sec)

mysql> Select UNIX_TIMESTAMP(NOW()) AS 'NUMBER OF SECONDS';

+-------------------+
| NUMBER OF SECONDS |
+-------------------+
|        1509248856 |
+-------------------+
1 row in set (0.00 sec)

mysql> Select UNIX_TIMESTAMP(Dateofreg) AS 'NUMBER OF SECONDS' from
       testing where StudentName = 'Gaurav';

+-------------------+
| NUMBER OF SECONDS |
+-------------------+
|        1509247113 |
+-------------------+
1 row in set (0.00 sec)