MariaDB [test]> CREATE TABLE t1 (dt DATETIME NOT NULL);

Query OK, 0 rows affected (0.29 sec)

MariaDB [test]> INSERT INTO t1 VALUES (NOW()),(NOW());

Query OK, 2 rows affected (0.06 sec)

Records: 2  Duplicates: 0  Warnings: 0

MariaDB [test]> set sql_mode='';

Query OK, 0 rows affected (0.00 sec)

MariaDB [test]> explain  SELECT * FROM t1 WHERE concat( dt, '2012-12-21 12:12:12' ) IS NULL;

+------+-------------+-------+------+---------------+------+---------+------+------+------------------+

| id   | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra            |

+------+-------------+-------+------+---------------+------+---------+------+------+------------------+

|    1 | SIMPLE      | NULL  | NULL | NULL          | NULL | NULL    | NULL | NULL | Impossible WHERE |

+------+-------------+-------+------+---------------+------+---------+------+------+------------------+

1 row in set (0.00 sec)

MariaDB [test]> set sql_mode='STRICT_TRANS_TABLES';

Query OK, 0 rows affected (0.00 sec)

MariaDB [test]> explain  SELECT * FROM t1 WHERE concat( dt, '2012-12-21 12:12:12' ) IS NULL;

+------+-------------+-------+------+---------------+------+---------+------+------+-------------+

| id   | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra       |

+------+-------------+-------+------+---------------+------+---------+------+------+-------------+

|    1 | SIMPLE      | t1    | ALL  | NULL          | NULL | NULL    | NULL |    2 | Using where |

+------+-------------+-------+------+---------------+------+---------+------+------+-------------+

1 row in set (0.00 sec)