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 mysql> select * from user2; 2 +----+-------+-----+----------+-----+ 3 | id | name | age | address | sex | 4 +----+-------+-----+----------+-----+ 5 | 1 | brand | 21 | fuzhou | 1 | 6 | 2 | helen | 20 | quanzhou | 0 | 7 | 3 | sol | 21 | xiamen | 0 | 8 +----+-------+-----+----------+-----+ 9 3 rows in set 10 11 mysql> select * from user2 where name=‘helen‘; 12 +----+-------+-----+----------+-----+ 13 | id | name | age | address | sex | 14 +----+-------+-----+----------+-----+ 15 | 2 | helen | 20 | quanzhou | 0 | 16 +----+-------+-----+----------+-----+ 17 1 row in set 18 19 mysql> select * from user2 where age=21; 20 +----+-------+-----+---------+-----+ 21 | id | name | age | address | sex | 22 +----+-------+-----+---------+-----+ 23 | 1 | brand | 21 | fuzhou | 1 | 24 | 3 | sol | 21 | xiamen | 0 | 25 +----+-------+-----+---------+-----+ 26 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：
select cname1,cname2,... from tname where cname <> cval; 或 select cname1,cname2,... from tname where cname != cval;
1 mysql> select * from user2; 2 +----+-------+-----+----------+-----+ 3 | id | name | age | address | sex | 4 +----+-------+-----+----------+-----+ 5 | 1 | brand | 21 | fuzhou | 1 | 6 | 2 | helen | 20 | quanzhou | 0 | 7 | 3 | sol | 21 | xiamen | 0 | 8 +----+-------+-----+----------+-----+ 9 3 rows in set 10 11 mysql> select * from user2 where age<>20; 12 +----+-------+-----+---------+-----+ 13 | id | name | age | address | sex | 14 +----+-------+-----+---------+-----+ 15 | 1 | brand | 21 | fuzhou | 1 | 16 | 3 | sol | 21 | xiamen | 0 | 17 +----+-------+-----+---------+-----+ 18 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 select cname1,cname2,... from tname where cname > cval; 2 3 select cname1,cname2,... from tname where cname < cval; 4 5 select cname1,cname2,... from tname where cname >= cval; 6 7 select cname1,cname2,... from tname where cname <= cval;
1 mysql> select * from user2 where age>20; 2 +----+-------+-----+---------+-----+ 3 | id | name | age | address | sex | 4 +----+-------+-----+---------+-----+ 5 | 1 | brand | 21 | fuzhou | 1 | 6 | 3 | sol | 21 | xiamen | 0 | 7 +----+-------+-----+---------+-----+ 8 2 rows in set 9 10 mysql> select * from user2 where age>=20; 11 +----+-------+-----+----------+-----+ 12 | id | name | age | address | sex | 13 +----+-------+-----+----------+-----+ 14 | 1 | brand | 21 | fuzhou | 1 | 15 | 2 | helen | 20 | quanzhou | 0 | 16 | 3 | sol | 21 | xiamen | 0 | 17 +----+-------+-----+----------+-----+ 18 3 rows in set 19 20 mysql> select * from user2 where age<21; 21 +----+-------+-----+----------+-----+ 22 | id | name | age | address | sex | 23 +----+-------+-----+----------+-----+ 24 | 2 | helen | 20 | quanzhou | 0 | 25 +----+-------+-----+----------+-----+ 26 1 row in set 27 28 mysql> select * from user2 where age<=21; 29 +----+-------+-----+----------+-----+ 30 | id | name | age | address | sex | 31 +----+-------+-----+----------+-----+ 32 | 1 | brand | 21 | fuzhou | 1 | 33 | 2 | helen | 20 | quanzhou | 0 | 34 | 3 | sol | 21 | xiamen | 0 | 35 +----+-------+-----+----------+-----+ 36 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 mysql> select * from user2; 2 +----+-------+-----+----------+-----+ 3 | id | name | age | address | sex | 4 +----+-------+-----+----------+-----+ 5 | 1 | brand | 21 | fuzhou | 1 | 6 | 2 | helen | 20 | quanzhou | 0 | 7 | 3 | sol | 21 | xiamen | 0 | 8 | 4 | weng | 33 | guizhou | 1 | 9 +----+-------+-----+----------+-----+ 10 4 rows in set 11 12 mysql> select * from user2 where age >20 and sex=1; 13 +----+-------+-----+---------+-----+ 14 | id | name | age | address | sex | 15 +----+-------+-----+---------+-----+ 16 | 1 | brand | 21 | fuzhou | 1 | 17 | 4 | weng | 33 | guizhou | 1 | 18 +----+-------+-----+---------+-----+ 19 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 mysql> select * from user2; 2 +----+-------+-----+----------+-----+ 3 | id | name | age | address | sex | 4 +----+-------+-----+----------+-----+ 5 | 1 | brand | 21 | fuzhou | 1 | 6 | 2 | helen | 20 | quanzhou | 0 | 7 | 3 | sol | 21 | xiamen | 0 | 8 | 4 | weng | 33 | guizhou | 1 | 9 +----+-------+-----+----------+-----+ 10 4 rows in set 11 12 mysql> select * from user2 where age>21 or age<21; 13 +----+-------+-----+----------+-----+ 14 | id | name | age | address | sex | 15 +----+-------+-----+----------+-----+ 16 | 2 | helen | 20 | quanzhou | 0 | 17 | 4 | weng | 33 | guizhou | 1 | 18 +----+-------+-----+----------+-----+ 19 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 mysql> select * from user2; 2 +----+-------+-----+----------+-----+ 3 | id | name | age | address | sex | 4 +----+-------+-----+----------+-----+ 5 | 1 | brand | 21 | fuzhou | 1 | 6 | 2 | helen | 20 | quanzhou | 0 | 7 | 3 | sol | 21 | xiamen | 0 | 8 | 4 | weng | 33 | guizhou | 1 | 9 +----+-------+-----+----------+-----+ 10 4 rows in set 11 12 mysql> select * from user2 where not(age>20); 13 +----+-------+-----+----------+-----+ 14 | id | name | age | address | sex | 15 +----+-------+-----+----------+-----+ 16 | 2 | helen | 20 | quanzhou | 0 | 17 +----+-------+-----+----------+-----+ 18 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 mysql> select * from user2; 2 +----+--------+-----+----------+-----+ 3 | id | name | age | address | sex | 4 +----+--------+-----+----------+-----+ 5 | 1 | brand | 21 | fuzhou | 1 | 6 | 2 | helen | 20 | quanzhou | 0 | 7 | 3 | sol | 21 | xiamen | 0 | 8 | 4 | weng | 33 | guizhou | 1 | 9 | 5 | selina | 25 | *g | 0 | 10 +----+--------+-----+----------+-----+ 11 5 rows in set 12 13 mysql> select * from user2 where name like ‘s%‘; 14 +----+--------+-----+---------+-----+ 15 | id | name | age | address | sex | 16 +----+--------+-----+---------+-----+ 17 | 3 | sol | 21 | xiamen | 0 | 18 | 5 | selina | 25 | *g | 0 | 19 +----+--------+-----+---------+-----+ 20 2 rows in set
_的使用
1 mysql> select * from user2; 2 +----+--------+-----+----------+-----+ 3 | id | name | age | address | sex | 4 +----+--------+-----+----------+-----+ 5 | 1 | brand | 21 | fuzhou | 1 | 6 | 2 | helen | 20 | quanzhou | 0 | 7 | 3 | sol | 21 | xiamen | 0 | 8 | 4 | weng | 33 | guizhou | 1 | 9 | 5 | selina | 25 | *g | 0 | 10 +----+--------+-----+----------+-----+ 11 5 rows in set 12 13 mysql> select * from user2 where name like ‘s_l‘; 14 +----+------+-----+---------+-----+ 15 | id | name | age | address | sex | 16 +----+------+-----+---------+-----+ 17 | 3 | sol | 21 | xiamen | 0 | 18 +----+------+-----+---------+-----+ 19 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 selec cname1,cname2,... from tname where cname between val1 and val2; 2 等同于 3 selec cname1,cname2,... from tname where cname >= val1 and cname <= val2;
查询年龄在[21,25]之间的数据：
1 mysql> select * from user2; 2 +----+--------+-----+----------+-----+ 3 | id | name | age | address | sex | 4 +----+--------+-----+----------+-----+ 5 | 1 | brand | 21 | fuzhou | 1 | 6 | 2 | helen | 20 | quanzhou | 0 | 7 | 3 | sol | 21 | xiamen | 0 | 8 | 4 | weng | 33 | guizhou | 1 | 9 | 5 | selina | 25 | *g | 0 | 10 +----+--------+-----+----------+-----+ 11 5 rows in set 12 13 mysql> select * from user2 where age between 21 and 25; 14 +----+--------+-----+---------+-----+ 15 | id | name | age | address | sex | 16 +----+--------+-----+---------+-----+ 17 | 1 | brand | 21 | fuzhou | 1 | 18 | 3 | sol | 21 | xiamen | 0 | 19 | 5 | selina | 25 | *g | 0 | 20 +----+--------+-----+---------+-----+ 21 3 rows in set 22 23 mysql> select * from user2 where age >= 21 and age <= 25; 24 +----+--------+-----+---------+-----+ 25 | id | name | age | address | sex | 26 +----+--------+-----+---------+-----+ 27 | 1 | brand | 21 | fuzhou | 1 | 28 | 3 | sol | 21 | xiamen | 0 | 29 | 5 | selina | 25 | *g | 0 | 30 +----+--------+-----+---------+-----+ 31 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 mysql> select * from user2; 2 +----+--------+-----+----------+-----+ 3 | id | name | age | address | sex | 4 +----+--------+-----+----------+-----+ 5 | 1 | brand | 21 | fuzhou | 1 | 6 | 2 | helen | 20 | quanzhou | 0 | 7 | 3 | sol | 21 | xiamen | 0 | 8 | 4 | weng | 33 | guizhou | 1 | 9 | 5 | selina | 25 | *g | 0 | 10 +----+--------+-----+----------+-----+ 11 5 rows in set 12 13 mysql> select * from user2 where address in(‘fuzhou‘,‘xiamen‘); 14 +----+-------+-----+---------+-----+ 15 | id | name | age | address | sex | 16 +----+-------+-----+---------+-----+ 17 | 1 | brand | 21 | fuzhou | 1 | 18 | 3 | sol | 21 | xiamen | 0 | 19 +----+-------+-----+---------+-----+ 20 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 mysql> select * from user2; 2 +----+--------+-----+----------+-----+ 3 | id | name | age | address | sex | 4 +----+--------+-----+----------+-----+ 5 | 1 | brand | 21 | fuzhou | 1 | 6 | 2 | helen | 20 | quanzhou | 0 | 7 | 3 | sol | 21 | xiamen | 0 | 8 | 4 | weng | 33 | guizhou | 1 | 9 | 5 | selina | 25 | *g | 0 | 10 +----+--------+-----+----------+-----+ 11 5 rows in set 12 13 mysql> select * from user2 where address not in(‘fuzhou‘,‘quanzhou‘,‘xiamen‘); 14 +----+--------+-----+---------+-----+ 15 | id | name | age | address | sex | 16 +----+--------+-----+---------+-----+ 17 | 4 | weng | 33 | guizhou | 1 | 18 | 5 | selina | 25 | *g | 0 | 19 +----+--------+-----+---------+-----+ 20 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 select cname1,cname2,... from tname where cname is null; 2 或者 3 select cname1,cname2,... from tname where cname is not null;
1 mysql> select * from user2 where address is null; 2 +----+--------+-----+---------+-----+ 3 | id | name | age | address | sex | 4 +----+--------+-----+---------+-----+ 5 | 5 | selina | 25 | NULL | 0 | 6 +----+--------+-----+---------+-----+ 7 1 row in set 8 9 mysql> select * from user2 where address is not null; 10 +----+-------+-----+----------+-----+ 11 | id | name | age | address | sex | 12 +----+-------+-----+----------+-----+ 13 | 1 | brand | 21 | fuzhou | 1 | 14 | 2 | helen | 20 | quanzhou | 0 | 15 | 3 | sol | 21 | xiamen | 0 | 16 | 4 | weng | 33 | guizhou | 1 | 17 +----+-------+-----+----------+-----+ 18 4 rows in set
有一种关键字 <=>，可以包含对null值得判断，但是目前用的比较少了，有兴趣可以去查查，这边不赘述。
总结
1、like表达式中的%匹配一个到多个任意字符，_匹配一个任意字符
2、空值查询需要使用IS NULL或者IS NOT NULL，其他查询运算符对NULL值无效。即使%通配符可以匹配任何东西，也不能匹配值NULL的数据。
3、建议创建表的时候，表字段不设置空，给字段一个default 默认值。
4、MySQL支持使用NOT对IN 、BETWEEN 和EXISTS子句取反 。