您现在的位置是：网站首页> 编程资料编程资料

MySQL全面瓦解之查询的过滤条件详解_Mysql_

2023-05-27 369人已围观

简介 MySQL全面瓦解之查询的过滤条件详解_Mysql_

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:

 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  

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：

 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 | +----+--------+-----+----------+--
                
                

                

                                            
上一篇：MySQL查询重写插件的使用_Mysql_

                                                                
下一篇：阿里云服务器安装Mysql数据库的详细教程_Mysql_

相关内容

点击排行

本栏推荐

猜你喜欢

-六神源码网