导读
datetime列设置了NOT NULL约束，但查询条件IS NULL却能返回结果，奇怪吗？
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：
[email protected]> insert into t1(id) select 1; — 不指定dt列的值
[email protected]> insert into t1 select 2, now(); — 指定dt列的值为now()
[email protected]> insert into t1(id) select 3; — 不指定dt列的值
查询数据：
[email protected]> 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：
[email protected]> 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)
[email protected]> 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.
文档出自：12.3.2 Comparison Functions and Operators, https://dev.mysql.com/doc/refman/5.7/en/comparison-operators.html
啊，对了，MySQL版本是 5.7.21。
