关于null值
比较运算符中使用NULL
例子：
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)
结论：任何值和NULL使用运算符（>、<、>=、<=、!=、<>）或者（in、not in、any/some、all）比较时，返回值都为NULL,NULL作为布尔值的时候，不为0也不为1
准备测试数据
mysql> create table test1(a int,b int);
Query OK, 0 rows affected (0.07 sec)
mysql> insert into test1 values(1,1),(1,null),(null,null);
Query OK, 3 rows affected (0.41 sec)
Records: 3 Duplicates: 0 Warnings: 0
mysql> select * from test1;
+------+------+
| a | b |
+------+------+
| 1 | 1 |
| 1 | NULL |
| NULL | NULL |
+------+------+
3 rows in set (0.00 sec)
上面有3条数据，注意NULL值的记录
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 (0.26 sec)
当in和null比较时，无法查询出null的记录
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)
当NOT IN后面有空值时，不论什么情况，整个sql的查询结果都为空
EXISTS、NOT EXISTS和NULL比较
mysql> select * from test1;
+------+------+
| a | b |
+------+------+
| 1 | 1 |
| 1 | NULL |
| NULL | NULL |
+------+------+
3 rows in set (0.00 sec)
-- 复制了test1表创建了test2表
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 (0.01 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)
看上面的效果，返回的结果为1或者0。
结论：判断是否为空只能用IS NULL、IS NOT NULL。
聚合函数中的NULL
示例1
mysql> select * from test1;
+------+------+
| a | b |
+------+------+
| 1 | 1 |
| 1 | NULL |
| NULL | NULL |
+------+------+
3 rows in set (0.00 sec)
mysql> select count(a),count(b),count(*) from test1;
+----------+----------+----------+
| count(a) | count(b) | count(*) |
+----------+----------+----------+
| 2 | 1 | 3 |
+----------+----------+----------+
1 row in set (0.26 sec)
count(a)返回了2行记录，a字段为NULL的没有统计出来
count(b)返回了1行记录，为NULL的2行记录没有统计出来
count(*)可以统计所有数据，不论字段的数据是否为NULL
示例2
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)
mysql> select count(*) from test1 where a is null;
+----------+
| count(*) |
+----------+
| 1 |
+----------+
1 row in set (0.00 sec)
上面第1个sql使用is null查询出了结果，第2个sql中count(a)返回的是0行。
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 (0.24 sec)
ysql> insert into test3 values(null,1);
ERROR 1048 (23000): Column 'a' cannot be null
上面创建了一个test3表，字段并未设置not null,当插入一条NULL的数据时，报错说a字段不能为NULL。
查询一下建表语句：
mysql> show create table test3\G
*************************** 1. row ***************************
Table: test3
Create Table: CREATE TABLE `test3` (
`a` int(11) NOT NULL,
`b` int(11) DEFAULT NULL,
PRIMARY KEY (`a`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4
1 row in set (0.00 sec)
从建表语句可以看到，当字段设为主键时，字段会自动被加上not null
结论：当字段设为主键时，字段会自动被加上not null
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
- 除非非常熟悉NULL特性，否则建议创建字段的时候字段不允许为NULL，给个默认值
