Question

我学到了Access不允许你进行完全外部连接的困难方式，在阅读如何模拟一个时，我已经开始明白如何这样做，但是我在尝试将此应用于只有两张桌子。

是否像执行以下操作一样简单？

SELECT * FROM table1
LEFT JOIN table2 ON table1.field = table2.field
LEFT JOIN table3 ON table1.field = table3.field
UNION
SELECT * FROM table1
RIGHT JOIN table2 ON table1.field = table2.field
RIGHT JOIN table3 ON table1.field = table3.field

Answer 1

对网络的快速搜索显示，在Stack Overflow和其他地方已多次讨论模拟FULL OUTER JOIN（＆＃34; FOJ＆＃34;）的主题，但答案似乎都局限于只有两个表的情况。我怀疑这是因为三张或更多桌子上的FOJ是

不常见，
可能很乱。

尽管如此，我还以为我试一试，这就是我想出的：

在两个表上模拟FOJ最常提到的技术是UNION ALL三种可能的情况：

In_Table2  In_Table1
---------  ---------
false      true
true       false
true       true

或代替true = 1和false = 0

In_Table2  In_Table1
---------  ---------
0          1
1          0
1          1

看起来像一个两位整数，有三个可能的非零值，而提取它们的SQL将是

Table2 RIGHT JOIN Table1 WHERE Table2.something IS NULL
UNION ALL
Table2 LEFT JOIN Table1 WHERE Table1.something IS NULL
UNION ALL
Table2 INNER JOIN Table1

具有三个表的情况的状态表因此看起来像一个三位整数，具有七个可能的非零值

In_Table3  In_Table2  In_Table1
---------  ---------  ---------
0          0          1
0          1          0
0          1          1
1          0          0
1          0          1
1          1          0
1          1          1

我想在整数值上进行连接，所以我只为七行中的每一行按顺序分配它们

Table3  Table2  Table1
------  ------  ------
                1
        2       
        3       3
4               
5               5
6       6       
7       7       7

所以我的测试表是

[表1]

n  txt 
-  ----
1  t1_1
3  t1_3
5  t1_5
7  t1_7

[表2]

n  txt 
-  ----
2  t2_2
3  t2_3
6  t2_6
7  t2_7

[表3]

n  txt 
-  ----
4  t3_4
5  t3_5
6  t3_6
7  t3_7

我在Access中创建了一个名为[foj12]的已保存查询，以执行[Table1]和[Table2]之间的FOJ

    SELECT t1.n AS t1_n, t1.txt AS t1_txt, t2.n AS t2_n, t2.txt AS t2_txt
    FROM
        Table1 t1
        INNER JOIN
        Table2 t2
            ON t1.n = t2.n
UNION ALL
    SELECT t1.n AS t1_n, t1.txt AS t1_txt, t2.n AS t2_n, t2.txt AS t2_txt
    FROM
        Table1 t1
        LEFT JOIN
        Table2 t2
            ON t1.n = t2.n
    WHERE t2.n IS NULL
UNION ALL 
    SELECT t1.n AS t1_n, t1.txt AS t1_txt, t2.n AS t2_n, t2.txt AS t2_txt
    FROM
        Table1 t1
        RIGHT JOIN
        Table2 t2
            ON t1.n = t2.n
    WHERE t1.n IS NULL

返回

t1_n  t1_txt  t2_n  t2_txt
----  ------  ----  ------
   1  t1_1                
                 2  t2_2  
   3  t1_3       3  t2_3  
   5  t1_5                
                 6  t2_6  
   7  t1_7       7  t2_7

然后我开始研究涉及[表3]的案例。它们如下，并且对应于＆＃34;联合＆＃34;在下面的SQL语句中查询。

表3匹配表1和表2（n = 7）
表3匹配表1但不匹配表2（n = 5）
表3匹配表2但不匹配表1（n = 6）
行与表1或表2中的任何内容不匹配（n = 4）
行（表1，表2）与表3没有任何共同之处（n = 1,2,3）

    SELECT f.t1_n, f.t1_txt, f.t2_n, f.t2_txt, t3.n AS t3_n, t3.txt AS t3_txt
    FROM
        Table3 t3
        INNER JOIN
        foj12 f
            ON t3.n = f.t1_n AND t3.n = f.t2_n
UNION ALL
    SELECT f.t1_n, f.t1_txt, f.t2_n, f.t2_txt, t3.n AS t3_n, t3.txt AS t3_txt
    FROM
        Table3 t3
        INNER JOIN
        foj12 f
            ON t3.n = f.t1_n
    WHERE f.t2_n IS NULL
UNION ALL
    SELECT f.t1_n, f.t1_txt, f.t2_n, f.t2_txt, t3.n AS t3_n, t3.txt AS t3_txt
    FROM
        Table3 t3
        INNER JOIN
        foj12 f
            ON t3.n = f.t2_n
    WHERE f.t1_n IS NULL
UNION ALL
    SELECT NULL, NULL, NULL, NULL, t3.n AS t3_n, t3.txt AS t3_txt
    FROM
        Table3 t3
    WHERE t3.n NOT IN (SELECT t1_n FROM foj12 WHERE t1_n IS NOT NULL)
        AND t3.n NOT IN (SELECT t2_n FROM foj12 WHERE t2_n IS NOT NULL)
UNION ALL
    SELECT f.t1_n, f.t1_txt, f.t2_n, f.t2_txt, NULL, NULL
    FROM foj12 f
    WHERE 
        (f.t1_n NOT IN (SELECT n FROM Table3) AND f.t2_n NOT IN (SELECT n FROM Table3))
        OR
        (f.t1_n NOT IN (SELECT n FROM Table3) AND f.t2_n IS NULL)
        OR
        (f.t1_n IS NULL AND f.t2_n NOT IN (SELECT n FROM Table3))
ORDER BY 5, 3, 1

那个小小的美丽回归

t1_n  t1_txt  t2_n  t2_txt  t3_n  t3_txt
----  ------  ----  ------  ----  ------
   1  t1_1                              
                 2  t2_2                
   3  t1_3       3  t2_3                
                               4  t3_4  
   5  t1_5                     5  t3_5  
                 6  t2_6       6  t3_6  
   7  t1_7       7  t2_7       7  t3_7

（毋庸置疑，我对添加第四张桌子并不感兴趣！）

欢迎评论。

在两个以上的表上使用Access模拟FULL OUTER JOIN

1 个答案: