从业以来主要在做客户端,用到的数据库都是表结构比较简单的 SQLite,以我那还给老师一大半的 SQL 水平倒也能对付。现在偶尔需要到后台的 SQL Server 里追查一些数据问题,就显得有点捉襟见肘了,特别是各种 JOIN,有时候傻傻分不清楚,于是索性弄明白并做个记录。

前言

在各种问答社区里谈及 SQL 里的各种 JOIN 之间的区别时,最被广为引用的是 CodeProject 上 C.L. Moffatt 的文章 Visual Representation of SQL Joins,他确实讲得简单明了,使用文氏图来帮助理解,效果明显。本文将沿用他的讲解方式,稍有演绎,可以视为该文较为粗糙的中译版。

约定

下文将使用两个数据库表 Table_A 和 Table_B 来进行示例讲解,其结构与数据分别如下:

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mysql> SELECT * FROM Table_A ORDER BY PK ASC;
+----+---------+
| PK | Value |
+----+---------+
| 1 | both ab |
| 2 | only a |
+----+---------+
2 rows in set (0.00 sec)

mysql> SELECT * from Table_B ORDER BY PK ASC;
+----+---------+
| PK | Value |
+----+---------+
| 1 | both ab |
| 3 | only b |
+----+---------+
2 rows in set (0.00 sec)

其中 PK 为 1 的记录在 Table_A 和 Table_B 中都有,2 为 Table_A 特有,3 为 Table_B 特有。

常用的 JOIN

INNER JOIN

INNER JOIN 一般被译作内连接。内连接查询能将左表(表 A)和右表(表 B)中能关联起来的数据连接后返回。

文氏图:

INNER JOIN

示例查询:

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SELECT A.PK AS A_PK, B.PK AS B_PK,
A.Value AS A_Value, B.Value AS B_Value
FROM Table_A A
INNER JOIN Table_B B
ON A.PK = B.PK;

查询结果:

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+------+------+---------+---------+
| A_PK | B_PK | A_Value | B_Value |
+------+------+---------+---------+
| 1 | 1 | both ab | both ab |
+------+------+---------+---------+
1 row in set (0.00 sec)

注:其中 ATable_A 的别名,BTable_B 的别名,下同。

LEFT JOIN

LEFT JOIN 一般被译作左连接,也写作 LEFT OUTER JOIN。左连接查询会返回左表(表 A)中所有记录,不管右表(表 B)中有没有关联的数据。在右表中找到的关联数据列也会被一起返回。

文氏图:

LEFT JOIN

示例查询:

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SELECT A.PK AS A_PK, B.PK AS B_PK,
A.Value AS A_Value, B.Value AS B_Value
FROM Table_A A
LEFT JOIN Table_B B
ON A.PK = B.PK;

查询结果:

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+------+------+---------+---------+
| A_PK | B_PK | A_Value | B_Value |
+------+------+---------+---------+
| 1 | 1 | both ab | both ba |
| 2 | NULL | only a | NULL |
+------+------+---------+---------+
2 rows in set (0.00 sec)

RIGHT JOIN

RIGHT JOIN 一般被译作右连接,也写作 RIGHT OUTER JOIN。右连接查询会返回右表(表 B)中所有记录,不管左表(表 A)中有没有关联的数据。在左表中找到的关联数据列也会被一起返回。

文氏图:

RIGHT JOIN

示例查询:

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SELECT A.PK AS A_PK, B.PK AS B_PK,
A.Value AS A_Value, B.Value AS B_Value
FROM Table_A A
RIGHT JOIN Table_B B
ON A.PK = B.PK;

查询结果:

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+------+------+---------+---------+
| A_PK | B_PK | A_Value | B_Value |
+------+------+---------+---------+
| 1 | 1 | both ab | both ba |
| NULL | 3 | NULL | only b |
+------+------+---------+---------+
2 rows in set (0.00 sec)

FULL OUTER JOIN

FULL OUTER JOIN 一般被译作外连接、全连接,实际查询语句中可以写作 FULL OUTER JOINFULL JOIN。外连接查询能返回左右表里的所有记录,其中左右表里能关联起来的记录被连接后返回。

文氏图:

FULL OUTER JOIN

示例查询:

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SELECT A.PK AS A_PK, B.PK AS B_PK,
A.Value AS A_Value, B.Value AS B_Value
FROM Table_A A
FULL OUTER JOIN Table_B B
ON A.PK = B.PK;

查询结果:

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ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'FULL OUTER JOIN Table_B B
ON A.PK = B.PK' at line 4

注:我当前示例使用的 MySQL 不支持 FULL OUTER JOIN

应当返回的结果(使用 UNION 模拟):

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mysql> SELECT * 
-> FROM Table_A
-> LEFT JOIN Table_B
-> ON Table_A.PK = Table_B.PK
-> UNION ALL
-> SELECT *
-> FROM Table_A
-> RIGHT JOIN Table_B
-> ON Table_A.PK = Table_B.PK
-> WHERE Table_A.PK IS NULL;
+------+---------+------+---------+
| PK | Value | PK | Value |
+------+---------+------+---------+
| 1 | both ab | 1 | both ba |
| 2 | only a | NULL | NULL |
| NULL | NULL | 3 | only b |
+------+---------+------+---------+
3 rows in set (0.00 sec)

小结

以上四种,就是 SQL 里常见 JOIN 的种类和概念了,看一下它们的合影:

有没有感觉少了些什么,学数学集合时完全不止这几种情况?确实如此,继续看。

延伸用法

LEFT JOIN EXCLUDING INNER JOIN

返回左表有但右表没有关联数据的记录集。

文氏图:

LEFT JOIN EXCLUDING INNER JOIN

示例查询:

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SELECT A.PK AS A_PK, B.PK AS B_PK,
A.Value AS A_Value, B.Value AS B_Value
FROM Table_A A
LEFT JOIN Table_B B
ON A.PK = B.PK
WHERE B.PK IS NULL;

查询结果:

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+------+------+---------+---------+
| A_PK | B_PK | A_Value | B_Value |
+------+------+---------+---------+
| 2 | NULL | only a | NULL |
+------+------+---------+---------+
1 row in set (0.01 sec)

RIGHT JOIN EXCLUDING INNER JOIN

返回右表有但左表没有关联数据的记录集。

文氏图:

RIGHT JOIN EXCLUDING INNER JOIN

示例查询:

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SELECT A.PK AS A_PK, B.PK AS B_PK,
A.Value AS A_Value, B.Value AS B_Value
FROM Table_A A
RIGHT JOIN Table_B B
ON A.PK = B.PK
WHERE A.PK IS NULL;

查询结果:

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+------+------+---------+---------+
| A_PK | B_PK | A_Value | B_Value |
+------+------+---------+---------+
| NULL | 3 | NULL | only b |
+------+------+---------+---------+
1 row in set (0.00 sec)

FULL OUTER JOIN EXCLUDING INNER JOIN

返回左表和右表里没有相互关联的记录集。

文氏图:

FULL OUTER JOIN EXCLUDING INNER JOIN

示例查询:

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SELECT A.PK AS A_PK, B.PK AS B_PK,
A.Value AS A_Value, B.Value AS B_Value
FROM Table_A A
FULL OUTER JOIN Table_B B
ON A.PK = B.PK
WHERE A.PK IS NULL
OR B.PK IS NULL;

因为使用到了 FULL OUTER JOIN,MySQL 在执行该查询时再次报错。

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ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'FULL OUTER JOIN Table_B B
ON A.PK = B.PK
WHERE A.PK IS NULL
OR B.PK IS NULL' at line 4

应当返回的结果(用 UNION 模拟):

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mysql> SELECT * 
-> FROM Table_A
-> LEFT JOIN Table_B
-> ON Table_A.PK = Table_B.PK
-> WHERE Table_B.PK IS NULL
-> UNION ALL
-> SELECT *
-> FROM Table_A
-> RIGHT JOIN Table_B
-> ON Table_A.PK = Table_B.PK
-> WHERE Table_A.PK IS NULL;
+------+--------+------+--------+
| PK | Value | PK | Value |
+------+--------+------+--------+
| 2 | only a | NULL | NULL |
| NULL | NULL | 3 | only b |
+------+--------+------+--------+
2 rows in set (0.00 sec)

总结

以上七种用法基本上可以覆盖各种 JOIN 查询了。七种用法的全家福:

SQL JOINS

看着它们,我仿佛回到了当年学数学,求交集并集的时代……

顺带张贴一下 C.L. Moffatt 带 SQL 语句的图片,配合学习,风味更佳:

SQL JOINS

更新:更多的 JOIN

除以上几种外,还有更多的 JOIN 用法,比如 CROSS JOIN(迪卡尔集)、SELF JOIN,可以参考 SQL JOINS Slide Presentation 学习。

CROSS JOIN

返回左表与右表之间符合条件的记录的迪卡尔集。

图示:

CORSS JOIN

示例查询:

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SELECT A.PK AS A_PK, B.PK AS B_PK,
A.Value AS A_Value, B.Value AS B_Value
FROM Table_A A
CROSS JOIN Table_B B;

查询结果:

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+------+------+---------+---------+
| A_PK | B_PK | A_Value | B_Value |
+------+------+---------+---------+
| 1 | 1 | both ab | both ba |
| 2 | 1 | only a | both ba |
| 1 | 3 | both ab | only b |
| 2 | 3 | only a | only b |
+------+------+---------+---------+
4 rows in set (0.00 sec)

上面讲过的几种 JOIN 查询的结果都可以用 CROSS JOIN 加条件模拟出来,比如 INNER JOIN 对应 CROSS JOIN ... WHERE A.PK = B.PK

SELF JOIN

返回表与自己连接后符合条件的记录,一般用在表里有一个字段是用主键作为外键的情况。

比如 Table_C 的结构与数据如下:

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+--------+----------+-------------+
| EMP_ID | EMP_NAME | EMP_SUPV_ID |
+--------+----------+-------------+
| 1001 | Ma | NULL |
| 1002 | Zhuang | 1001 |
+--------+----------+-------------+
2 rows in set (0.00 sec)

EMP_ID 字段表示员工 ID,EMP_NAME 字段表示员工姓名,EMP_SUPV_ID 表示主管 ID。

示例查询:

现在我们想查询所有有主管的员工及其对应的主管 ID 和姓名,就可以用 SELF JOIN 来实现。

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SELECT A.EMP_ID AS EMP_ID, A.EMP_NAME AS EMP_NAME, 
B.EMP_ID AS EMP_SUPV_ID, B.EMP_NAME AS EMP_SUPV_NAME
FROM Table_C A, Table_C B
WHERE A.EMP_SUPV_ID = B.EMP_ID;

查询结果:

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+--------+----------+-------------+---------------+
| EMP_ID | EMP_NAME | EMP_SUPV_ID | EMP_SUPV_NAME |
+--------+----------+-------------+---------------+
| 1002 | Zhuang | 1001 | Ma |
+--------+----------+-------------+---------------+
1 row in set (0.00 sec)

补充说明

  1. 文中的图使用 Keynote 绘制;

  2. 个人的体会是 SQL 里的 JOIN 查询与数学里的求交集、并集等很像;

  3. SQLite 不支持 RIGHT JOIN 和 FULL OUTER JOIN,可以使用 LEFT JOIN 和 UNION 来达到相同的效果;

  4. MySQL 不支持 FULL OUTER JOIN,可以使用 LEFT JOIN 和 UNION 来达到相同的效果;

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参考