select Null value in upper or lower in Order by clause in MySQL

Null value occurred at the last position

select rental_id,return_Date from rental order by return_date is null, return_Date desc 

rental_id|return_Date        |
---------+-------------------+
    16005|2005-09-02 02:35:22|
    16040|2005-09-02 02:19:33|
    15971|2005-09-02 01:28:33|
    15928|2005-09-01 23:43:24|
    15922|2005-09-01 22:27:31|
    15977|2005-09-01 22:12:10|
    15982|2005-09-01 21:51:31|
    15973|2005-09-01 20:08:41|
    15834|2005-09-01 19:31:50|
    15911|2005-09-01 19:14:53|

Null value present at the upper position

select rental_id,return_Date from rental order by return_date is not null, return_Date desc;

rental_id|return_Date        |
---------+-------------------+
    14734|                   |
    14741|                   |
    14760|                   |
    14769|                   |
    14878|                   |
    14915|                   |
    14928|                   |
    14933|                   |

Default Null value at the first position in Mysql

select * from rental order by return_Date asc

rental_id|return_Date        |
---------+-------------------+
    11496|                   |
    11541|                   |
    11563|                   |
    11577|                   |
    11593|                   |
    11611|                   |
    11646|                   |