MySQL全面瓦解7:查询的过滤条件

发布于 2022-8-1 19:23
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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:

MySQL全面瓦解7:查询的过滤条件-开源基础软件社区

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;
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

MySQL全面瓦解7:查询的过滤条件-开源基础软件社区

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 。

 

 

文章转载自公众号:架构与思维

已于2022-8-1 19:23:01修改
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