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 * fromuser2;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 * fromuser2;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
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 whereage>=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 * fromuser2;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 * fromuser2;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 * fromuser2;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 * fromuser2;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 | taiwang | 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 | taiwang | 0 |
19 +----+--------+-----+---------+-----+
20 2 rows in set
_的使用
1 mysql> select * fromuser2;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 | taiwang | 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 andval2;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 * fromuser2;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 | taiwang | 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 | taiwang | 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 | taiwang | 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 * fromuser2;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 | taiwang | 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 * fromuser2;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 | taiwang | 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 | taiwang | 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 。