If all the values in MySQL COALESCE() function are NULL then it returns NULL as the output. It means that this function does not find any non-NULL value in the list.
Example
mysql> Select COALESCE(NULL, NULL, NULL, NULL); +----------------------------------+ | COALESCE(NULL, NULL, NULL, NULL) | +----------------------------------+ | NULL | +----------------------------------+ 1 row in set (0.00 sec)