Mysql NULL导致的神坑

比较运算符中使用NULL

mysql> select 1>NULL;+--------+| 1>NULL |+--------+|  NULL |+--------+1 row in set (0.00 sec)mysql> select 1<NULL;+--------+| 1<NULL |+--------+|  NULL |+--------+1 row in set (0.00 sec)mysql> select 1<>NULL;+---------+| 1<>NULL |+---------+|  NULL |+---------+1 row in set (0.00 sec)mysql> select 1>NULL;+--------+| 1>NULL |+--------+|  NULL |+--------+1 row in set (0.00 sec)mysql> select 1<NULL;+--------+| 1<NULL |+--------+|  NULL |+--------+1 row in set (0.00 sec)mysql> select 1>=NULL;+---------+| 1>=NULL |+---------+|  NULL |+---------+1 row in set (0.00 sec)mysql> select 1<=NULL;+---------+| 1<=NULL |+---------+|  NULL |+---------+1 row in set (0.00 sec)mysql> select 1!=NULL;+---------+| 1!=NULL |+---------+|  NULL |+---------+1 row in set (0.00 sec)mysql> select 1<>NULL;+---------+| 1<>NULL |+---------+|  NULL |+---------+1 row in set (0.00 sec)mysql> select NULL=NULL,NULL!=NULL;+-----------+------------+| NULL=NULL | NULL!=NULL |+-----------+------------+|   NULL |    NULL |+-----------+------------+1 row in set (0.00 sec)mysql> select 1 in (null),1 not in (null),null in (null),null not in (null);+-------------+-----------------+----------------+--------------------+| 1 in (null) | 1 not in (null) | null in (null) | null not in (null) |+-------------+-----------------+----------------+--------------------+|    NULL |      NULL |      NULL |        NULL |+-------------+-----------------+----------------+--------------------+1 row in set (0.00 sec)mysql> select 1=any(select null),null=any(select null);+--------------------+-----------------------+| 1=any(select null) | null=any(select null) |+--------------------+-----------------------+|        NULL |         NULL |+--------------------+-----------------------+1 row in set (0.00 sec)mysql> select 1=all(select null),null=all(select null);+--------------------+-----------------------+| 1=all(select null) | null=all(select null) |+--------------------+-----------------------+|        NULL |         NULL |+--------------------+-----------------------+1 row in set (0.00 sec)

准备数据

mysql> create table test1(a int,b int);Query OK, 0 rows affected (0.01 sec)mysql> insert into test1 values (1,1),(1,null),(null,null);Query OK, 3 rows affected (0.00 sec)Records: 3 Duplicates: 0 Warnings: 0mysql> select * from test1;+------+------+| a  | b  |+------+------+|  1 |  1 ||  1 | NULL || NULL | NULL |+------+------+3 rows in set (0.00 sec)

IN、NOT IN和NULL比较

IN和NULL比较

mysql> select * from test1;+------+------+| a  | b  |+------+------+|  1 |  1 ||  1 | NULL || NULL | NULL |+------+------+3 rows in set (0.00 sec)mysql> select * from test1 where a in (null);Empty set (0.00 sec)mysql> select * from test1 where a in (null,1);+------+------+| a  | b  |+------+------+|  1 |  1 ||  1 | NULL |+------+------+2 rows in set (0.00 sec)

NOT IN 和NULL比较

mysql> select * from test1 where a not in (1);Empty set (0.00 sec)mysql> select * from test1 where a not in (null);Empty set (0.00 sec)mysql> select * from test1 where a not in (null,2);Empty set (0.00 sec)mysql> select * from test1 where a not in (2);+------+------+| a  | b  |+------+------+|  1 |  1 ||  1 | NULL |+------+------+2 rows in set (0.00 sec)

EXISTS、NOT EXISTS和NULL比较

mysql> select * from test2;+------+------+| a  | b  |+------+------+|  1 |  1 ||  1 | NULL || NULL | NULL |+------+------+3 rows in set (0.00 sec)mysql> select * from test1 t1 where exists (select * from test2 t2 where t1.a = t2.a);+------+------+| a  | b  |+------+------+|  1 |  1 ||  1 | NULL |+------+------+2 rows in set (0.00 sec)mysql> select * from test1 t1 where not exists (select * from test2 t2 where t1.a = t2.a);+------+------+| a  | b  |+------+------+| NULL | NULL |+------+------+1 row in set (0.00 sec)

查询语句中使用exists、not exists对比test1.a=test2.a，因为=不能比较NULL，结果和预期一致。

判断NULL只能用IS NULL、IS NOT NULL

mysql> select 1 is not null;+---------------+| 1 is not null |+---------------+|       1 |+---------------+1 row in set (0.00 sec)mysql> select 1 is null;+-----------+| 1 is null |+-----------+|     0 |+-----------+1 row in set (0.00 sec)mysql> select null is null;+--------------+| null is null |+--------------+|      1 |+--------------+1 row in set (0.00 sec)mysql> select null is not null;+------------------+| null is not null |+------------------+|        0 |+------------------+1 row in set (0.00 sec)

结论：判断是否为空只能用IS NULL、IS NOT NULL。

聚合函数中NULL的坑

示例

mysql> select count(a),count(b),count(*) from test1;+----------+----------+----------+| count(a) | count(b) | count(*) |+----------+----------+----------+|    2 |    1 |    3 |+----------+----------+----------+1 row in set (0.00 sec)

mysql> select * from test1 where a is null;+------+------+| a  | b  |+------+------+| NULL | NULL |+------+------+1 row in set (0.00 sec)mysql> select count(a) from test1 where a is null;+----------+| count(a) |+----------+|    0 |+----------+1 row in set (0.00 sec)

结论：count(字段)无法统计字段为NULL的值，count(*)可以统计值为null的行。

NULL不能作为主键的值

mysql> create table test3(a int primary key,b int);Query OK, 0 rows affected (0.01 sec)mysql> insert into test3 values (null,1);ERROR 1048 (23000): Column 'a' cannot be null

mysql> show create table test3;+-------+------------+| Table | Create Table   |+-------+------------+| test3 | CREATE TABLE `test3` ( `a` int(11) NOT NULL, `b` int(11) DEFAULT NULL, PRIMARY KEY (`a`)) ENGINE=InnoDB DEFAULT CHARSET=utf8+-------+------------+1 row in set (0.00 sec)

结论：当字段为主键的时候，字段会自动设置为not null。

看了上面这些还是比较晕，NULL的情况确实比较难以处理，容易出错，最有效的方法就是避免使用NULL。所以，强烈建议创建字段的时候字段不允许为NULL，设置一个默认值。

总结

NULL作为布尔值的时候，不为1也不为0
任何值和NULL使用运算符（>、<、>=、<=、!=、<>）或者（in、not in、any/some、all），返回值都为NULL
当IN和NULL比较时，无法查询出为NULL的记录
当NOT IN 后面有NULL值时，不论什么情况下，整个sql的查询结果都为空
判断是否为空只能用IS NULL、IS NOT NULL
count(字段)无法统计字段为NULL的值，count(*)可以统计值为null的行
当字段为主键的时候，字段会自动设置为not null
NULL导致的坑让人防不胜防，强烈建议创建字段的时候字段不允许为NULL，给个默认值

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