What will MySQL return if we use the UNIX_TIMESTAMP() function without parameters?

In this case, MySQL returns a Unix timestamp of the current date and time. Therefore, we can say that using no argument is the same as using NOW() as argument to the UNIX_TIMESTAMP() function.

For example, if we run a UNIX_TIMESTAMP() query with no value and using NOW() as parameter MySQL returns the same results.

mysql> Select UNIX_TIMESTAMP();
+------------------+
| UNIX_TIMESTAMP() |
+------------------+
|       1509405559 |
+------------------+
1 row in set (0.00 sec)

mysql> Select UNIX_TIMESTAMP(NOW());
+-----------------------+
| UNIX_TIMESTAMP(NOW()) |
+-----------------------+
|            1509405559 |
+-----------------------+
1 row in set (0.00 sec)

This can be verified by passing the results of the above query as a parameter to the FROM_UNIXTIME() function. MySQL will return the current timestamp value as shown below -

mysql> Select FROM_UNIXTIME(1509405559);
+---------------------------+
| FROM_UNIXTIME(1509405559) |
+---------------------------+
| 2017-10-31 04:49:19       |
+---------------------------+
1 row in set (0.00 sec)

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