MySQL全面瓦解7:查询的过滤条件

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MySQL全面瓦解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:

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 | 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 * 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 | 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 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 | 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 * 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 | 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 * 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 | 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

IN

BETWEEN
















EXISTS 子句取反 。








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