NOT NULL列用IS NULL也能查到数据？

测试表DDL

CREATE TABLE `t1` (
 `id` int(11) DEFAULT NULL,
 `dt` datetime NOT NULL DEFAULT '0000-00-00 00:00:00'
) ENGINE=InnoDB;

插入测试数据：

yejr@imysql.com> insert into t1(id) select 1; --- 不指定dt列的值

yejr@imysql.com> insert into t1 select 2, now(); --- 指定dt列的值为now()

yejr@imysql.com> insert into t1(id) select 3; --- 不指定dt列的值

查询数据：

yejr@imysql.com> select * from t1 where dt is null;
+------+---------------------+
| id | dt |
+------+---------------------+
| 1 | 0000-00-00 00:00:00 |
| 3 | 0000-00-00 00:00:00 |
+------+---------------------+
2 rows in set (0.00 sec)

有没有觉得很奇怪，为什么查到了2条 dt 列值为 '0000-00-00 00:00:00' 的记录？

先查看执行计划：

yejr@imysql.com> desc select * from t1 where dt is null\G
*************************** 1. row ***************************
 id: 1
 select_type: SIMPLE
 table: t2
 partitions: NULL
 type: ALL
possible_keys: NULL
 key: NULL
 key_len: NULL
 ref: NULL
 rows: 5
 filtered: 20.00
 Extra: Using where
1 row in set, 1 warning (0.00 sec)

yejr@imysql.com> show warnings\G
*************************** 1. row ***************************
 Level: Note
 Code: 1003
Message: /* select#1 */ select `yejr`.`t1`.`id` AS `id`,`yejr`.`t2`.`dt` AS `dt` from `yejr`.`t1` where (`yejr`.`t1`.`dt` = '0000-00-00 00:00:00')

发现 IS NULL 条件被转换了，所以才能查到结果，这是为什么呢？ 我尝试了调整SQL_MODE，发现并没什么卵用，最后还是在官方文档找到了答案：

For DATE and DATETIME columns that are declared as NOT NULL, you can find the special date '0000-00-00' by using a statement like this:

SELECT * FROM tbl_name WHERE date_column IS NULL

This is needed to get some ODBC applications to work because ODBC does not support a '0000-00-00' date value.

See Obtaining Auto-Increment Values, and the description for the FLAG_AUTO_IS_NULL option at Connector/ODBC Connection Parameters.