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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:
? 1select 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：
? 1select cname1,cname2,... from tname where cname = cval;
?
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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 3select cname1,cname2,... from tname where cname <> cval;
或
select cname1,cname2,... from tname where cname != cval;
?
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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 7select 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;
?
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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
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：
? 1select cname1,cname2,... from tname where cname1 operators cval1 and cname2 operators cval2
?
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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：
? 1select cname1,cname2,... from tname where cname1 operators cval1 or cname2 operators cval2
?
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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：
? 1select cname1,cname2,... from tname where not(cname operators cval)
?
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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。
? 1select 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 20mysql> 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 19mysql> 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 3selec 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 31mysql> 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：
? 1select cname1,cname2,... from tname where cname in (val1,val2,...);
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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 20mysql> 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 3select cname1,cname2,... from tname where cname is null;
或者
select cname1,cname2,... from tname where cname is not null;
?
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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
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 。
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原文链接：https://www.cnblogs.com/wzh2010/p/13843027.html