关于mysql的null相关查询的一些坑

我就是我 2023-08-17 16:57 149阅读 0赞

我们先看一下效果，然后在解释，示例如下：

mysql> create table test5 (a int not null,b int,c varchar(10)); Query OK, 0 rows affected (0.01 sec) mysql> insert into test5 values (1,2,'a'),(3,null,'b'),(4,5,null); Query OK, 3 rows affected (0.01 sec) Records: 3 Duplicates: 0 Warnings: 0 mysql> select * from test5; +---+------+------+ | a | b | c | +---+------+------+ | 1 | 2 | a | | 3 | NULL | b | | 4 | 5 | NULL | +---+------+------+ 3 rows in set (0.00 sec)

上面我们创建了一个表test5，3个字段，a不能为空，b、c可以为空，插入了3条数据，睁大眼睛看效果了：

mysql> select * from test5 where b>0; +---+------+------+ | a | b | c | +---+------+------+ | 1 | 2 | a | | 4 | 5 | NULL | +---+------+------+ 2 rows in set (0.00 sec) mysql> select * from test5 where b<=0; Empty set (0.00 sec) mysql> select * from test5 where b=NULL; Empty set (0.00 sec) mysql> select * from test5 t where t.b between 0 and 100; +---+------+------+ | a | b | c | +---+------+------+ | 1 | 2 | a | | 4 | 5 | NULL | +---+------+------+ 2 rows in set (0.00 sec) mysql> select * from test5 where c like '%'; +---+------+------+ | a | b | c | +---+------+------+ | 1 | 2 | a | | 3 | NULL | b | +---+------+------+ 2 rows in set (0.00 sec) mysql> select * from test5 where c in ('a','b',NULL); +---+------+------+ | a | b | c | +---+------+------+ | 1 | 2 | a | | 3 | NULL | b | +---+------+------+ 2 rows in set (0.00 sec) mysql> select * from test5 where c not in ('a','b',NULL); Empty set (0.00 sec)

认真看一下上面的查询：

上面带有条件的查询，对字段b进行条件查询的，b的值为NULL的都没有出现。

对c字段进行like '%'查询、in、not查询，c中为NULL的记录始终没有查询出来。

between and查询，为空的记录也没有查询出来。

**结论：查询运算符、like、between and、in、not in对NULL值查询不起效。**

那NULL如何查询呢？继续向下看

### IS NULL/IS NOT NULL（NULL值专用查询） ###

上面介绍的各种运算符对NULL值均不起效，mysql为我们提供了查询空值的语法：IS NULL、IS NOT NULL。

#### IS NULL（返回值为空的记录） ####

select 列名 from 表名 where 列 is null;

> 查询指定的列的值为NULL的记录。

如：

mysql> create table test7 (a int,b varchar(10)); Query OK, 0 rows affected (0.01 sec) mysql> insert into test7 (a,b) values (1,'a'),(null,'b'),(3,null),(null,null),(4,'c'); Query OK, 5 rows affected (0.00 sec) Records: 5 Duplicates: 0 Warnings: 0 mysql> select * from test7; +------+------+ | a | b | +------+------+ | 1 | a | | NULL | b | | 3 | NULL | | NULL | NULL | | 4 | c | +------+------+ 5 rows in set (0.00 sec) mysql> select * from test7 t where t.a is null; +------+------+ | a | b | +------+------+ | NULL | b | | NULL | NULL | +------+------+ 2 rows in set (0.00 sec) mysql> select * from test7 t where t.a is null or t.b is null; +------+------+ | a | b | +------+------+ | NULL | b | | 3 | NULL | | NULL | NULL | +------+------+ 3 rows in set (0.00 sec)

#### IS NULL（返回值不为空的记录） ####

select 列名 from 表名 where 列 is not null;

> 查询指定的列的值不为NULL的记录。

如：

mysql> select * from test7 t where t.a is not null; +------+------+ | a | b | +------+------+ | 1 | a | | 3 | NULL | | 4 | c | +------+------+ 3 rows in set (0.00 sec) mysql> select * from test7 t where t.a is not null and t.b is not null; +------+------+ | a | b | +------+------+ | 1 | a | | 4 | c | +------+------+ 2 rows in set (0.00 sec)

转载于:https://www.cnblogs.com/mr-wuxiansheng/p/11578881.html