mysql> select account_id from branches where extension="8002034138";
+------------+
| account_id |
+------------+
|       NULL |
+------------+
1 row in set (0.01 sec)

mysql> select * from branches where account_id=NULL;
Empty set (0.01 sec)

mysql> select * from branches where account_id='NULL';
Empty set (0.01 sec)

mysql>