登录
首页 >  数据库 >  MySQL

Mysql - JOIN详解

来源:SegmentFault

时间:2023-01-10 18:37:17 292浏览 收藏

知识点掌握了,还需要不断练习才能熟练运用。下面golang学习网给大家带来一个数据库开发实战,手把手教大家学习《Mysql - JOIN详解》,在实现功能的过程中也带大家重新温习相关知识点,温故而知新,回头看看说不定又有不一样的感悟!

0 索引

  • JOIN语句的执行顺序
  • INNER/LEFT/RIGHT/FULL JOIN的区别
  • ONWHERE的区别

1 概述

一个完整的SQL语句中会被拆分成多个子句,子句的执行过程中会产生虚拟表(vt),但是结果只返回最后一张虚拟表。从这个思路出发,我们试着理解一下JOIN查询的执行过程并解答一些常见的问题。
如果之前对不同JOIN的执行结果没有概念,可以结合这篇文章往下看

2 JOIN的执行顺序

以下是JOIN查询的通用结构

SELECT  
  FROM  JOIN  
      ON  
        WHERE 

它的执行顺序如下(SQL语句里第一个被执行的总是FROM子句)

  • FROM:对左右两张表执行笛卡尔积,产生第一张表vt1。行数为n*m(n为左表的行数,m为右表的行数
  • ON:根据ON的条件逐行筛选vt1,将结果插入vt2中
  • JOIN:添加外部行,如果指定了LEFT JOIN(LEFT OUTER JOIN),则先遍历一遍左表的每一行,其中不在vt2的行会被插入到vt2,该行的剩余字段将被填充为NULL,形成vt3;如果指定了RIGHT JOIN也是同理。但如果指定的是INNER JOIN,则不会添加外部行,上述插入过程被忽略,vt2=vt3(所以INNER JOIN的过滤条件放在ONWHERE里 执行结果是没有区别的,下文会细说)
  • WHERE:对vt3进行条件过滤,满足条件的行被输出到vt4
  • SELECT:取出vt4的指定字段到vt5

下面用一个例子介绍一下上述联表的过程(这个例子不是个好的实践,只是为了说明join语法)

3 举例

创建一个用户信息表:

CREATE TABLE `user_info` (
  `userid` int(11) NOT NULL,
  `name` varchar(255) NOT NULL,
  UNIQUE `userid` (`userid`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4

再创建一个用户余额表:

CREATE TABLE `user_account` (
  `userid` int(11) NOT NULL,
  `money` bigint(20) NOT NULL,
 UNIQUE `userid` (`userid`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4

随便导入一些数据:

select * from user_info;
+--------+------+
| userid | name |
+--------+------+
|   1001 | x    |
|   1002 | y    |
|   1003 | z    |
|   1004 | a    |
|   1005 | b    |
|   1006 | c    |
|   1007 | d    |
|   1008 | e    |
+--------+------+
8 rows in set (0.00 sec)

select * from user_account;
+--------+-------+
| userid | money |
+--------+-------+
|   1001 |    22 |
|   1002 |    30 |
|   1003 |     8 |
|   1009 |    11 |
+--------+-------+
4 rows in set (0.00 sec)

一共8个用户有用户名,4个用户的账户有余额。
取出userid为1003的用户姓名和余额,SQL如下

SELECT i.name, a.money 
  FROM user_info as i 
    LEFT JOIN user_account as a 
      ON i.userid = a.userid 
        WHERE a.userid = 1003;

第一步:执行FROM子句对两张表进行笛卡尔积操作

笛卡尔积操作后会返回两张表中所有行的组合,左表user_info有8行,右表user_account有4行,生成的虚拟表vt1就是8*4=32行:

SELECT * FROM user_info as i LEFT JOIN user_account as a ON 1;
+--------+------+--------+-------+
| userid | name | userid | money |
+--------+------+--------+-------+
|   1001 | x    |   1001 |    22 |
|   1002 | y    |   1001 |    22 |
|   1003 | z    |   1001 |    22 |
|   1004 | a    |   1001 |    22 |
|   1005 | b    |   1001 |    22 |
|   1006 | c    |   1001 |    22 |
|   1007 | d    |   1001 |    22 |
|   1008 | e    |   1001 |    22 |
|   1001 | x    |   1002 |    30 |
|   1002 | y    |   1002 |    30 |
|   1003 | z    |   1002 |    30 |
|   1004 | a    |   1002 |    30 |
|   1005 | b    |   1002 |    30 |
|   1006 | c    |   1002 |    30 |
|   1007 | d    |   1002 |    30 |
|   1008 | e    |   1002 |    30 |
|   1001 | x    |   1003 |     8 |
|   1002 | y    |   1003 |     8 |
|   1003 | z    |   1003 |     8 |
|   1004 | a    |   1003 |     8 |
|   1005 | b    |   1003 |     8 |
|   1006 | c    |   1003 |     8 |
|   1007 | d    |   1003 |     8 |
|   1008 | e    |   1003 |     8 |
|   1001 | x    |   1009 |    11 |
|   1002 | y    |   1009 |    11 |
|   1003 | z    |   1009 |    11 |
|   1004 | a    |   1009 |    11 |
|   1005 | b    |   1009 |    11 |
|   1006 | c    |   1009 |    11 |
|   1007 | d    |   1009 |    11 |
|   1008 | e    |   1009 |    11 |
+--------+------+--------+-------+
32 rows in set (0.00 sec)

第二步:执行ON子句过滤掉不满足条件的行

ON i.userid = a.userid 过滤之后vt2如下:

+--------+------+--------+-------+
| userid | name | userid | money |
+--------+------+--------+-------+
|   1001 | x    |   1001 |    22 |
|   1002 | y    |   1002 |    30 |
|   1003 | z    |   1003 |     8 |
+--------+------+--------+-------+

第三步:JOIN 添加外部行

LEFT JOIN会将左表未出现在vt2的行插入进vt2,每一行的剩余字段将被填充为NULL,RIGHT JOIN同理
本例中用的是LEFT JOIN,所以会将左表user_info剩下的行都添上 生成表vt3:

+--------+------+--------+-------+
| userid | name | userid | money |
+--------+------+--------+-------+
|   1001 | x    |   1001 |    22 |
|   1002 | y    |   1002 |    30 |
|   1003 | z    |   1003 |     8 |
|   1004 | a    |   NULL |  NULL |
|   1005 | b    |   NULL |  NULL |
|   1006 | c    |   NULL |  NULL |
|   1007 | d    |   NULL |  NULL |
|   1008 | e    |   NULL |  NULL |
+--------+------+--------+-------+

第四步:WHERE条件过滤

WHERE a.userid = 1003 生成表vt4:

+--------+------+--------+-------+
| userid | name | userid | money |
+--------+------+--------+-------+
|   1003 | z    |   1003 |     8 |
+--------+------+--------+-------+

第五步:SELECT

SELECT i.name, a.money 生成vt5:

+------+-------+
| name | money |
+------+-------+
| z    |     8 |
+------+-------+

虚拟表vt5作为最终结果返回给客户端

介绍完联表的过程之后,我们看看常用JOIN的区别

4 INNER/LEFT/RIGHT/FULL JOIN的区别

  • INNER JOIN...ON...: 返回 左右表互相匹配的所有行(因为只执行上文的第二步ON过滤,不执行第三步 添加外部行)
  • LEFT JOIN...ON...: 返回左表的所有行,若某些行在右表里没有相对应的匹配行,则将右表的列在新表中置为NULL
  • RIGHT JOIN...ON...: 返回右表的所有行,若某些行在左表里没有相对应的匹配行,则将左表的列在新表中置为NULL

INNER JOIN

拿上文的第三步添加外部行来举例,若LEFT JOIN替换成INNER JOIN,则会跳过这一步,生成的表vt3与vt2一模一样:

+--------+------+--------+-------+
| userid | name | userid | money |
+--------+------+--------+-------+
|   1001 | x    |   1001 |    22 |
|   1002 | y    |   1002 |    30 |
|   1003 | z    |   1003 |     8 |
+--------+------+--------+-------+

RIGHT JOIN

LEFT JOIN替换成RIGHT JOIN,则生成的表vt3如下:

+--------+------+--------+-------+
| userid | name | userid | money |
+--------+------+--------+-------+
|   1001 | x    |   1001 |    22 |
|   1002 | y    |   1002 |    30 |
|   1003 | z    |   1003 |     8 |
|   NULL | NULL |   1009 |    11 |
+--------+------+--------+-------+

因为user_account(右表)里存在userid=1009这一行,而user_info(左表)里却找不到这一行的记录,所以会在第三步插入以下一行:

|   NULL | NULL |   1009 |    11 |

FULL JOIN

上文引用的文章中提到了标准SQL定义的FULL JOIN,这在mysql里是不支持的,不过我们可以通过LEFT JOIN + UNION + RIGHT JOIN 来实现FULL JOIN

SELECT * 
  FROM user_info as i 
    RIGHT JOIN user_account as a 
      ON a.userid=i.userid
union 
SELECT * 
  FROM user_info as i 
    LEFT JOIN user_account as a 
      ON a.userid=i.userid;

他会返回如下结果:

+--------+------+--------+-------+
| userid | name | userid | money |
+--------+------+--------+-------+
|   1001 | x    |   1001 |    22 |
|   1002 | y    |   1002 |    30 |
|   1003 | z    |   1003 |     8 |
|   NULL | NULL |   1009 |    11 |
|   1004 | a    |   NULL |  NULL |
|   1005 | b    |   NULL |  NULL |
|   1006 | c    |   NULL |  NULL |
|   1007 | d    |   NULL |  NULL |
|   1008 | e    |   NULL |  NULL |
+--------+------+--------+-------+

ps:其实我们从语义上就能看出LEFT JOINRIGHT JOIN没什么差别,两者的结果差异取决于左右表的放置顺序,以下内容摘自mysql官方文档:

RIGHT JOIN works analogously to LEFT JOIN. To keep code portable across databases, it is recommended that you use LEFT JOIN instead of RIGHT JOIN.

所以当你纠结使用LEFT JOIN还是RIGHT JOIN时,尽可能只使用LEFT JOIN吧

5 ON和WHERE的区别

上文把JOIN的执行顺序了解清楚之后,ON和WHERE的区别也就很好理解了。
举例说明:

SELECT * 
  FROM user_info as i
    LEFT JOIN user_account as a
      ON i.userid = a.userid and i.userid = 1003;

SELECT * 
  FROM user_info as i
    LEFT JOIN user_account as a
      ON i.userid = a.userid where i.userid = 1003;

第一种情况LEFT JOIN在执行完第二步ON子句后,筛选出满足i.userid = a.userid and i.userid = 1003的行,生成表vt2,然后执行第三步JOIN子句,将外部行添加进虚拟表生成vt3即最终结果:

vt2:
+--------+------+--------+-------+
| userid | name | userid | money |
+--------+------+--------+-------+
|   1003 | z    |   1003 |     8 |
+--------+------+--------+-------+
vt3:
+--------+------+--------+-------+
| userid | name | userid | money |
+--------+------+--------+-------+
|   1001 | x    |   NULL |  NULL |
|   1002 | y    |   NULL |  NULL |
|   1003 | z    |   1003 |     8 |
|   1004 | a    |   NULL |  NULL |
|   1005 | b    |   NULL |  NULL |
|   1006 | c    |   NULL |  NULL |
|   1007 | d    |   NULL |  NULL |
|   1008 | e    |   NULL |  NULL |
+--------+------+--------+-------+

而第二种情况LEFT JOIN在执行完第二步ON子句后,筛选出满足i.userid = a.userid的行,生成表vt2;再执行第三步JOIN子句添加外部行生成表vt3;然后执行第四步WHERE子句,再对vt3表进行过滤生成vt4,得的最终结果:

vt2:
+--------+------+--------+-------+
| userid | name | userid | money |
+--------+------+--------+-------+
|   1001 | x    |   1001 |    22 |
|   1002 | y    |   1002 |    30 |
|   1003 | z    |   1003 |     8 |
+--------+------+--------+-------+
vt3:
+--------+------+--------+-------+
| userid | name | userid | money |
+--------+------+--------+-------+
|   1001 | x    |   1001 |    22 |
|   1002 | y    |   1002 |    30 |
|   1003 | z    |   1003 |     8 |
|   1004 | a    |   NULL |  NULL |
|   1005 | b    |   NULL |  NULL |
|   1006 | c    |   NULL |  NULL |
|   1007 | d    |   NULL |  NULL |
|   1008 | e    |   NULL |  NULL |
+--------+------+--------+-------+
vt4:
+--------+------+--------+-------+
| userid | name | userid | money |
+--------+------+--------+-------+
|   1003 | z    |   1003 |     8 |
+--------+------+--------+-------+

如果将上例的LEFT JOIN替换成INNER JOIN,不论将条件过滤放到ON还是WHERE里,结果都是一样的,因为INNER JOIN不会执行第三步添加外部行

SELECT * 
  FROM user_info as i
    INNER JOIN user_account as a
      ON i.userid = a.userid and i.userid = 1003;

SELECT * 
  FROM user_info as i
    INNER JOIN user_account as a
      ON i.userid = a.userid where i.userid = 1003;

返回结果都是:

+--------+------+--------+-------+
| userid | name | userid | money |
+--------+------+--------+-------+
|   1003 | z    |   1003 |     8 |
+--------+------+--------+-------+

参考资料

《MySQL技术内幕:SQL编程》
SQL Joins - W3Schools
sql - What is the difference between “INNER JOIN” and “OUTER JOIN”?
MySQL :: MySQL 8.0 Reference Manual :: 13.2.10.2 JOIN Syntax
Visual Representation of SQL Joins
Join (SQL) - Wikipedia)

以上就是本文的全部内容了,是否有顺利帮助你解决问题?若是能给你带来学习上的帮助,请大家多多支持golang学习网!更多关于数据库的相关知识,也可关注golang学习网公众号。

声明:本文转载于:SegmentFault 如有侵犯,请联系study_golang@163.com删除
相关阅读
更多>
最新阅读
更多>
课程推荐
更多>
评论列表