数据库 —— 常见的9种连接方式|8月更文挑战

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1 内连接(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

内连接(INNER-JOIN).png

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)

注意:其中A为Table_A的别名,B为Table_B的别名,下同。

2 左连接(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

左连接(LEFT-JOIN).png

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 右连接(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

右连接(RIGHT-JOIN).png

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 全连接(FULL JOIN)

FULL 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。(MySQL不支持FULL OUTER JOIN

文氏图

全连接(FULL-OUTER-JOIN).png

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

注意: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)

SQL常见缩影

SQL常用JOIN.png

5 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.png

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)

6 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.png

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)

7 FULL 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.png

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)

SQL所有JOIN

SQL所有JOIN.png

8 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

CROSS-JOIN.png

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
CROSS JOIN Table_B B;

查询结果

+------+------+---------+---------+
| 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。

9 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:

+--------+----------+-------------+
| 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。

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

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

  • 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