mysql-joins具体用法说明

编程学习 2021-07-05 14:37www.dzhlxh.cn编程入门
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JOIN对于接触过数据库的人,这个词都不陌生,而且很多人很清楚各种JOIN,还有很多人对这个理解也不是很透彻。
假设我们有两个表,Table_A和Table_B。这两个表中的数据如下所示:

TABLE_A										|		 TABLE_B
 PK Value     	 				|   PK Value
---- ----------    				|   ---- ----------
 1 FOX     				|   1 TROT
 2 COP     				|   2 CAR
 3 TAXI     				|   3 CAB
 6 WASHINGTON    				|   6 MONUMENT
 7 DELL     				|   7 PC
 5 ARIZONA    				|   8 MICROSOFT
 4 LINCOLN    				|   9 APPLE
 10 LUCENT     				|   11 SCOTCH

Join 语法:

join_table:
 table_reference JOIN table_factor [join_condition]											//内连接
 | table_reference {LEFT|RIGHT|FULL} [OUTER] JOIN table_reference join_condition				//外连接
 | table_reference LEFT SEMI JOIN table_reference join_condition								//左半连接
 | table_reference CROSS JOIN table_reference [join_condition] (as of Hive 0.10)


table_reference:
 table_factor					//表
 | join_table						//join语句


table_factor:
 tbl_name [alias]				//表名[别名]
 | table_subquery alias			//子查寻[别名]
 | ( table_references )			//带空号的table_reference


join_condition:
 ON expression					//on开头的条件语句

1、Inner JOIN: (内连接)


这是最简单、最容易理解的连接,也是最常见的连接。此查询将返回左表(表A)中具有右表(表B)中匹配记录的所有记录。此连接写成如下:

SELECT <select_list> 
FROM Table_A A
INNER JOIN Table_B B
ON A.Key = B.Key
-- Inner JOIN
SELECT A.PK AS A_PK, A.Value AS A_Value,
  B.Value AS B_Value, B.PK AS B_PK
FROM Table_A A
INNER JOIN Table_B B
ON A.PK = B.PK

A_PK A_Value B_Value B_PK
---- ---------- ---------- ----
 1 FOX  TROT   1
 2 COP  CAR   2
 3 TAXI  CAB   3
 6 WASHINGTON MONUMENT  6
 7 DELL  PC   7

(5 row(s) affected)

2、Left JOIN: (左连接)


此查询将返回左表(表A)中的所有记录,而不管这些记录是否与右表(表B)中的任何记录匹配。它还将从正确的表中返回任何匹配的记录。此连接写成如下:

SELECT <select_list>
FROM Table_A A
LEFT JOIN Table_B B
ON A.Key = B.Key
-- Left JOIN
SELECT A.PK AS A_PK, A.Value AS A_Value,
B.Value AS B_Value, B.PK AS B_PK
FROM Table_A A
LEFT JOIN Table_B B
ON A.PK = B.PK

A_PK A_Value B_Value B_PK
---- ---------- ---------- ----
 1 FOX  TROT   1
 2 COP  CAR   2
 3 TAXI  CAB   3
 4 LINCOLN NULL  NULL
 5 ARIZONA NULL  NULL
 6 WASHINGTON MONUMENT  6
 7 DELL  PC   7
 10 LUCENT  NULL  NULL

(8 row(s) affected)

3、Left Excluding JOIN: (左连接排除内连接结果)

此查询将返回左表(表A)中与右表(表B)中的任何记录都不匹配的所有记录。此连接写成如下:

SELECT <select_list> 
FROM Table_A A
LEFT JOIN Table_B B
ON A.Key = B.Key
WHERE B.Key IS NULL
-- Left Excluding JOIN
SELECT A.PK AS A_PK, A.Value AS A_Value,
B.Value AS B_Value, B.PK AS B_PK
FROM Table_A A
LEFT JOIN Table_B B
ON A.PK = B.PK
WHERE B.PK IS NULL

A_PK A_Value B_Value B_PK
---- ---------- ---------- ----
 4 LINCOLN NULL  NULL
 5 ARIZONA NULL  NULL
 10 LUCENT  NULL  NULL
(3 row(s) affected)

4、Right JOIN: (右连接)


此查询将返回右表(表B)中的所有记录,而不管这些记录中是否有任何记录与左表(表A)中的记录相匹配。它还将返回左表中的任何匹配记录。此连接写成如下:

SELECT <select_list>
FROM Table_A A
RIGHT JOIN Table_B B
ON A.Key = B.Key
-- Right JOIN
SELECT A.PK AS A_PK, A.Value AS A_Value,
B.Value AS B_Value, B.PK AS B_PK
FROM Table_A A
RIGHT JOIN Table_B B
ON A.PK = B.PK

A_PK A_Value B_Value B_PK
---- ---------- ---------- ----
 1 FOX  TROT   1
 2 COP  CAR   2
 3 TAXI  CAB   3
 6 WASHINGTON MONUMENT  6
 7 DELL  PC   7
NULL NULL  MICROSOFT  8
NULL NULL  APPLE   9
NULL NULL  SCOTCH  11

(8 row(s) affected)

5、Right Excluding JOIN: (右连接排除内连接结果)


此查询将返回右表(表B)中与左表(表A)中的任何记录都不匹配的所有记录。此连接写成如下:

SELECT <select_list>
FROM Table_A A
RIGHT JOIN Table_B B
ON A.Key = B.Key
WHERE A.Key IS NULL
-- Right Excluding JOIN
SELECT A.PK AS A_PK, A.Value AS A_Value,
B.Value AS B_Value, B.PK AS B_PK
FROM Table_A A
RIGHT JOIN Table_B B
ON A.PK = B.PK
WHERE A.PK IS NULL

A_PK A_Value B_Value B_PK
---- ---------- ---------- ----
NULL NULL  MICROSOFT  8
NULL NULL  APPLE   9
NULL NULL  SCOTCH  11

(3 row(s) affected)

6、Outer JOIN: (外连接)


此联接也可以称为完全外联接或完全联接。此查询将返回两个表中的所有记录,连接左表(表A)中与右表(表B)中的记录相匹配的记录。此连接写成如下:

SELECT <select_list>
FROM Table_A A
FULL OUTER JOIN Table_B B
ON A.Key = B.Key
-- Outer JOIN
SELECT A.PK AS A_PK, A.Value AS A_Value,
B.Value AS B_Value, B.PK AS B_PK
FROM Table_A A
FULL OUTER JOIN Table_B B
ON A.PK = B.PK

A_PK A_Value B_Value B_PK
---- ---------- ---------- ----
 1 FOX  TROT   1
 2 COP  CAR   2
 3 TAXI  CAB   3
 6 WASHINGTON MONUMENT  6
 7 DELL  PC   7
NULL NULL  MICROSOFT  8
NULL NULL  APPLE   9
NULL NULL  SCOTCH  11
 5 ARIZONA NULL  NULL
 4 LINCOLN NULL  NULL
 10 LUCENT  NULL  NULL

(11 row(s) affected)

7、Outer Excluding JOIN: (外连接排除内连接结果)


此查询将返回左表(表A)中的所有记录和右表(表B)中不匹配的所有记录。我还不需要使用这种类型的联接,但所有其他类型的联接我都相当频繁地使用。此连接写成如下:

SELECT <select_list>
FROM Table_A A
FULL OUTER JOIN Table_B B
ON A.Key = B.Key
WHERE A.Key IS NULL OR B.Key IS NULL
-- Outer Excluding JOIN
SELECT A.PK AS A_PK, A.Value AS A_Value,
B.Value AS B_Value, B.PK AS B_PK
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

A_PK A_Value B_Value B_PK
---- ---------- ---------- ----
NULL NULL  MICROSOFT  8
NULL NULL  APPLE   9
NULL NULL  SCOTCH  11
 5 ARIZONA NULL  NULL
 4 LINCOLN NULL  NULL
 10 LUCENT  NULL  NULL

(6 row(s) affected)

注意,在外部联接上,首先返回内部连接记录,然后返回右连接记录,最后返回左连接记录(至少,我的Microsoft SQL Server就是这样做的;当然,这不需要使用任何ORDERBY语句)。您可以访问维基百科文章以获得更多信息(但是,条目不是图形化的)。我还创建了一个备忘单,您可以在需要时打印出来。如果您右键单击狼蚁网站SEO优化的图像并选择“将目标保存为.”,您将下载完整大小的图像。

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