As we know that a NULL is no value and it is not the same as zero. MySQL represents a database column as NULL if it does not contain any data. Now, if we will use NULL in any arithmetic expression then the result will be NULL also.
Example
mysql> Select 65/NULL,65+NULL,65*NULL,65-NULL,65%NULL; +---------+---------+---------+---------+---------+ | 65/NULL | 65+NULL | 65*NULL | 65-NULL | 65%NULL | +---------+---------+---------+---------+---------+ | NULL | NULL | NULL | NULL | NULL | +---------+---------+---------+---------+---------+ 1 row in set (0.00 sec)
From the above example, it can be observed that if we will use NULL in an arithmetic expression then the result would be NULL itself.