MySQL从零到有7：查询的过滤条件

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：

关系运算基本的语法格式如下:

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：

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

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：

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

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：

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;

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： 

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

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：

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

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：

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

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。

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

%的使用

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

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：

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：

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：

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

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：

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。

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

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 。