MySQL全面瓦解之查询的过滤条件详解

吾爱主题阅读：162 2024-04-01 23:51:13 评论：0

概述

在实际的业务场景应用中，我们经常要根据业务条件获取并筛选出我们的目标数据。这个过程我们称之为数据查询的过滤。而过滤过程使用的各种条件（比如日期时间、用户、状态）是我们获取精准数据的必要步骤，

这样才能得到我们期望的结果。所以本章我们来学习MySQL中查询过滤条件的各种用法。

关系运算

关系运算就是where语句后跟上一个或者n个条件，满足where后面条件的数据会被返回，反之不满足的就会被过滤掉。operators指的是运算符，有如下几种情况：

运算符	说明
=	等于
<> 或者 !=	不等于
>	大于
>=	大于等于
<	小于
<=	小于等于

关系运算基本的语法格式如下:

1	`select` `cname1,cname2,...` `from` `tname` `where` `cname operators cval`

等于=

查询出列和后面的值严格相等的数据，非值类型的需要对后面值加上引号，值类型的不需要。

语法格式如下：

1	`select` `cname1,cname2,...` `from` `tname` `where` `cname = cval;`

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 mysql> select * from user2; + ----+-------+-----+----------+-----+ | id | name | age | address | sex | + ----+-------+-----+----------+-----+ | 1 | brand | 21 | fuzhou | 1 | | 2 | helen | 20 | quanzhou | 0 | | 3 | sol | 21 | xiamen | 0 | + ----+-------+-----+----------+-----+ 3 rows in set mysql> select * from user2 where name = 'helen' ; + ----+-------+-----+----------+-----+ | id | name | age | address | sex | + ----+-------+-----+----------+-----+ | 2 | helen | 20 | quanzhou | 0 | + ----+-------+-----+----------+-----+ 1 row in set mysql> select * from user2 where age=21; + ----+-------+-----+---------+-----+ | id | name | age | address | sex | + ----+-------+-----+---------+-----+ | 1 | brand | 21 | fuzhou | 1 | | 3 | sol | 21 | xiamen | 0 | + ----+-------+-----+---------+-----+ 2 rows in set

不等于（<>、!=）

不等于有两种写法，一种是<>，另一种是!=，意思一样，可随意切换使用，但是 <> 先于 != 出现，所以看很多以前的例子，<> 出现频率比较高，可移植性更强，推荐使用。

不等于的目的是查询出与条件不符和结果，格式如下：

1 2 3 select cname1,cname2,... from tname where cname <> cval; 或 select cname1,cname2,... from tname where cname != cval;

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 mysql> select * from user2; + ----+-------+-----+----------+-----+ | id | name | age | address | sex | + ----+-------+-----+----------+-----+ | 1 | brand | 21 | fuzhou | 1 | | 2 | helen | 20 | quanzhou | 0 | | 3 | sol | 21 | xiamen | 0 | + ----+-------+-----+----------+-----+ 3 rows in set mysql> select * from user2 where age<>20; + ----+-------+-----+---------+-----+ | id | name | age | address | sex | + ----+-------+-----+---------+-----+ | 1 | brand | 21 | fuzhou | 1 | | 3 | sol | 21 | xiamen | 0 | + ----+-------+-----+---------+-----+ 2 rows in set

大于小于（> <）

一般用于数值或者日期、时间类型的比较，格式如下：

1 2 3 4 5 6 7 select cname1,cname2,... from tname where cname > cval; select cname1,cname2,... from tname where cname < cval; select cname1,cname2,... from tname where cname >= cval; select cname1,cname2,... from tname where cname <= cval;

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 mysql> select * from user2 where age>20; + ----+-------+-----+---------+-----+ | id | name | age | address | sex | + ----+-------+-----+---------+-----+ | 1 | brand | 21 | fuzhou | 1 | | 3 | sol | 21 | xiamen | 0 | + ----+-------+-----+---------+-----+ 2 rows in set mysql> select * from user2 where age>=20; + ----+-------+-----+----------+-----+ | id | name | age | address | sex | + ----+-------+-----+----------+-----+ | 1 | brand | 21 | fuzhou | 1 | | 2 | helen | 20 | quanzhou | 0 | | 3 | sol | 21 | xiamen | 0 | + ----+-------+-----+----------+-----+ 3 rows in set mysql> select * from user2 where age<21; + ----+-------+-----+----------+-----+ | id | name | age | address | sex | + ----+-------+-----+----------+-----+ | 2 | helen | 20 | quanzhou | 0 | + ----+-------+-----+----------+-----+ 1 row in set mysql> select * from user2 where age<=21; + ----+-------+-----+----------+-----+ | id | name | age | address | sex | + ----+-------+-----+----------+-----+ | 1 | brand | 21 | fuzhou | 1 | | 2 | helen | 20 | quanzhou | 0 | | 3 | sol | 21 | xiamen | 0 | + ----+-------+-----+----------+-----+ 3 rows in set

逻辑运算

运算符	说明
AND	多个条件都成立
OR	多个条件中满足一个
NOT	对条件进行取非操作

AND（且）

当需要多个条件进行数据过滤的时候，使用这种方式，and的每个表达式都是要成立，过滤出来的数据就是用户需要的。

下面过滤出年龄和性别两个条件都成立的数据，语法格式如下：

1	`select` `cname1,cname2,...` `from` `tname` `where` `cname1 operators cval1` `and` `cname2 operators cval2`

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 mysql> select * from user2; + ----+-------+-----+----------+-----+ | id | name | age | address | sex | + ----+-------+-----+----------+-----+ | 1 | brand | 21 | fuzhou | 1 | | 2 | helen | 20 | quanzhou | 0 | | 3 | sol | 21 | xiamen | 0 | | 4 | weng | 33 | guizhou | 1 | + ----+-------+-----+----------+-----+ 4 rows in set mysql> select * from user2 where age >20 and sex=1; + ----+-------+-----+---------+-----+ | id | name | age | address | sex | + ----+-------+-----+---------+-----+ | 1 | brand | 21 | fuzhou | 1 | | 4 | weng | 33 | guizhou | 1 | + ----+-------+-----+---------+-----+ 2 rows in set

OR（或）

当多个条件中只要满足一个条件即进行数据过滤。

下面条件过滤出年龄大于21岁和小于21岁的数据，语法格式如下：

1	`select` `cname1,cname2,...` `from` `tname` `where` `cname1 operators cval1` `or` `cname2 operators cval2`

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 mysql> select * from user2; + ----+-------+-----+----------+-----+ | id | name | age | address | sex | + ----+-------+-----+----------+-----+ | 1 | brand | 21 | fuzhou | 1 | | 2 | helen | 20 | quanzhou | 0 | | 3 | sol | 21 | xiamen | 0 | | 4 | weng | 33 | guizhou | 1 | + ----+-------+-----+----------+-----+ 4 rows in set mysql> select * from user2 where age>21 or age<21; + ----+-------+-----+----------+-----+ | id | name | age | address | sex | + ----+-------+-----+----------+-----+ | 2 | helen | 20 | quanzhou | 0 | | 4 | weng | 33 | guizhou | 1 | + ----+-------+-----+----------+-----+ 2 rows in set

NOT（取非）

对某个满足的条件进行取反，过滤出来的数据就是用户需要的。

下面过滤不属于年龄大于20的数据，语法格式如下：

1	`select` `cname1,cname2,...` `from` `tname` `where` `not` `(cname operators cval)`

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 mysql> select * from user2; + ----+-------+-----+----------+-----+ | id | name | age | address | sex | + ----+-------+-----+----------+-----+ | 1 | brand | 21 | fuzhou | 1 | | 2 | helen | 20 | quanzhou | 0 | | 3 | sol | 21 | xiamen | 0 | | 4 | weng | 33 | guizhou | 1 | + ----+-------+-----+----------+-----+ 4 rows in set mysql> select * from user2 where not (age>20); + ----+-------+-----+----------+-----+ | id | name | age | address | sex | + ----+-------+-----+----------+-----+ | 2 | helen | 20 | quanzhou | 0 | + ----+-------+-----+----------+-----+ 1 row in set

模糊匹配

就像我们上面的那个用户表信息表（包含名称、年龄、地址、性别），当我们要查询名称为s开头的用户时，就可以用到 like 关键字了，他用以模糊匹配数据。

语法格式如下,pattern中可以包含通配符，有两种。%：表示匹配任意一个或n个字符； _：表示匹配任意一个字符。

1	`select` `cname1,cname2,...` `from` `tname` `where` `cname` `like` `pattern;`

%的使用

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 mysql> select * from user2; + ----+--------+-----+----------+-----+ | id | name | age | address | sex | + ----+--------+-----+----------+-----+ | 1 | brand | 21 | fuzhou | 1 | | 2 | helen | 20 | quanzhou | 0 | | 3 | sol | 21 | xiamen | 0 | | 4 | weng | 33 | guizhou | 1 | | 5 | selina | 25 | taiwang | 0 | + ----+--------+-----+----------+-----+ 5 rows in set mysql> select * from user2 where name like 's%' ; + ----+--------+-----+---------+-----+ | id | name | age | address | sex | + ----+--------+-----+---------+-----+ | 3 | sol | 21 | xiamen | 0 | | 5 | selina | 25 | taiwang | 0 | + ----+--------+-----+---------+-----+ 2 rows in set

_的使用

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 mysql> select * from user2; + ----+--------+-----+----------+-----+ | id | name | age | address | sex | + ----+--------+-----+----------+-----+ | 1 | brand | 21 | fuzhou | 1 | | 2 | helen | 20 | quanzhou | 0 | | 3 | sol | 21 | xiamen | 0 | | 4 | weng | 33 | guizhou | 1 | | 5 | selina | 25 | taiwang | 0 | + ----+--------+-----+----------+-----+ 5 rows in set mysql> select * from user2 where name like 's_l' ; + ----+------+-----+---------+-----+ | id | name | age | address | sex | + ----+------+-----+---------+-----+ | 3 | sol | 21 | xiamen | 0 | + ----+------+-----+---------+-----+ 1 row in set

注意点

1、不要过度使用模糊匹配得通配符。如果其他操作符能达到相同的目的，应该使用其他操作符

2、对大体量的表进行模糊匹配的时候尽量不要以%开头，比如 like '%username',这样会执行扫表，效率较慢。尽量明确模糊查找的开头部分，比如 like 'brand%'，会先定位到brand开头的数据，效率高很多。

范围值检查

BETWEEN AND(区间查询)

操作符 BETWEEN … AND 会选取介于两个值之间的数据范围，这些值可以是数值、文本或者日期，属于一个闭区间查询。

and 的左边val1 和右边 val2 分别表示两个临界值，等同于数学公式[val1,val2] ，属于这两个区间的数据会被过滤出来（>=val1 和 <=val2）,所以语法格式如下：

1 2 3 selec cname1,cname2,... from tname where cname between val1 and val2; 等同于 selec cname1,cname2,... from tname where cname >= val1 and cname <= val2;

查询年龄在[21,25]之间的数据：

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 mysql> select * from user2; + ----+--------+-----+----------+-----+ | id | name | age | address | sex | + ----+--------+-----+----------+-----+ | 1 | brand | 21 | fuzhou | 1 | | 2 | helen | 20 | quanzhou | 0 | | 3 | sol | 21 | xiamen | 0 | | 4 | weng | 33 | guizhou | 1 | | 5 | selina | 25 | taiwang | 0 | + ----+--------+-----+----------+-----+ 5 rows in set mysql> select * from user2 where age between 21 and 25; + ----+--------+-----+---------+-----+ | id | name | age | address | sex | + ----+--------+-----+---------+-----+ | 1 | brand | 21 | fuzhou | 1 | | 3 | sol | 21 | xiamen | 0 | | 5 | selina | 25 | taiwang | 0 | + ----+--------+-----+---------+-----+ 3 rows in set mysql> select * from user2 where age >= 21 and age <= 25; + ----+--------+-----+---------+-----+ | id | name | age | address | sex | + ----+--------+-----+---------+-----+ | 1 | brand | 21 | fuzhou | 1 | | 3 | sol | 21 | xiamen | 0 | | 5 | selina | 25 | taiwang | 0 | + ----+--------+-----+---------+-----+ 3 rows in set

IN（包含查询）

按照上面得数据，如果我们想查出居住地位于福州和厦门得用户数据，应该使用 IN操作符，因为 IN 操作符允许我们在 WHERE 子句中指定多个值，符合这些值中得某一项，既满足条件返回数据。

语法格式如下，in 后面列表的值类型必须一致或兼容，且不支持通配符：

1	`select` `cname1,cname2,...` `from` `tname` `where` `cname` `in` `(val1,val2,...);`

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 mysql> select * from user2; + ----+--------+-----+----------+-----+ | id | name | age | address | sex | + ----+--------+-----+----------+-----+ | 1 | brand | 21 | fuzhou | 1 | | 2 | helen | 20 | quanzhou | 0 | | 3 | sol | 21 | xiamen | 0 | | 4 | weng | 33 | guizhou | 1 | | 5 | selina | 25 | taiwang | 0 | + ----+--------+-----+----------+-----+ 5 rows in set mysql> select * from user2 where address in ( 'fuzhou' , 'xiamen' ); + ----+-------+-----+---------+-----+ | id | name | age | address | sex | + ----+-------+-----+---------+-----+ | 1 | brand | 21 | fuzhou | 1 | | 3 | sol | 21 | xiamen | 0 | + ----+-------+-----+---------+-----+ 2 rows in set

NOT IN（对包含查询取反）

我们上面已经学习过了not得用户，对not后面执行得表达式进行取反得操作，测试下：

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 mysql> select * from user2; + ----+--------+-----+----------+-----+ | id | name | age | address | sex | + ----+--------+-----+----------+-----+ | 1 | brand | 21 | fuzhou | 1 | | 2 | helen | 20 | quanzhou | 0 | | 3 | sol | 21 | xiamen | 0 | | 4 | weng | 33 | guizhou | 1 | | 5 | selina | 25 | taiwang | 0 | + ----+--------+-----+----------+-----+ 5 rows in set mysql> select * from user2 where address not in ( 'fuzhou' , 'quanzhou' , 'xiamen' ); + ----+--------+-----+---------+-----+ | id | name | age | address | sex | + ----+--------+-----+---------+-----+ | 4 | weng | 33 | guizhou | 1 | | 5 | selina | 25 | taiwang | 0 | + ----+--------+-----+---------+-----+ 2 rows in set

空值检查

IS NULL/IS NOT NULL

判断是否为空，语法格式如下，这边注意的是，对值为null的数据，各种比较运算符、like、between and、in、not in查询都不起作用，只有is null 能够过滤出来。

1 2 3 select cname1,cname2,... from tname where cname is null ; 或者 select cname1,cname2,... from tname where cname is not null ;

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 mysql> select * from user2 where address is null ; + ----+--------+-----+---------+-----+ | id | name | age | address | sex | + ----+--------+-----+---------+-----+ | 5 | selina | 25 | NULL | 0 | + ----+--------+-----+---------+-----+ 1 row in set mysql> select * from user2 where address is not null ; + ----+-------+-----+----------+-----+ | id | name | age | address | sex | + ----+-------+-----+----------+-----+ | 1 | brand | 21 | fuzhou | 1 | | 2 | helen | 20 | quanzhou | 0 | | 3 | sol | 21 | xiamen | 0 | | 4 | weng | 33 | guizhou | 1 | + ----+-------+-----+----------+-----+ 4 rows in set