mysql>
mysql> SELECT CONCAT('a','b'), CONCAT('a',NULL,'b');
+-----------------+----------------------+
| CONCAT('a','b') | CONCAT('a',NULL,'b') |
+-----------------+----------------------+
| ab | NULL |
+-----------------+----------------------+
1 row in set (0.00 sec)
mysql>
But not all functions behave that way. CONCAT_WS() (concatenate with separator) simply ignores NULL arguments entirely:
mysql>
mysql> SELECT CONCAT_WS('/','a','b'), CONCAT_WS('/','a',NULL,'b');
+------------------------+-----------------------------+
| CONCAT_WS('/','a','b') | CONCAT_WS('/','a',NULL,'b') |
+------------------------+-----------------------------+
| a/b | a/b |
+------------------------+-----------------------------+
1 row in set (0.00 sec)
mysql>