图解数据库中的各种 JOIN

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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：

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：

1、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：

示例查询：

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：

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

注：其中 A 为 Table_A 的别名，B 为 Table_B 的别名，下同。

2、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：

示例查询：

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：

+------+------+---------+---------+
| 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)

3、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：

示例查询：

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：

+------+------+---------+---------+
| 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)

4、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 JOIN 或 FULL 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：

示例查询：

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：

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）：

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：

延伸用法

1、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：

示例查询：

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：

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

2、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：

示例查询：

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：

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

3、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：

示例查询：

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。

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）：

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：

补充说明

文中的图使用 Keynote 绘制；

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；

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；

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