MySQL CONCAT() function will return a NULL if you will add a NULL value while linking two strings. Following example will demonstrate it −
Example
mysql> Select CONCAT('Tutorials',NULL,'Point');
+----------------------------------+
| CONCAT('Tutorials',NULL,'Point') |
+----------------------------------+
| NULL |
+----------------------------------+
1 row in set (0.06 sec)
mysql> Select CONCAT('TutorialsPoint','.com',NULL);
+--------------------------------------+
| CONCAT('TutorialsPoint','.com',NULL) |
+--------------------------------------+
| NULL |
+--------------------------------------+
1 row in set (0.00 sec)