在兩個以上的表上模擬完全外部連接並訪問

Question

我學會了Access不允許完全外部連接的難題，在閱讀如何模擬外部連接時，我已經開始瞭解如何做到這一點，但我有問題試圖應用這不僅僅是兩個表。

就像做下面的事情一樣簡單嗎？

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

快速搜索網頁的顯示，模擬完全外部的主題JOIN（「FOJ」）在堆棧溢出或其他地方已經討論過很多次，但答案似乎都限制自己的情況只有兩張桌子。我懷疑這是因爲三個或多個表FOJ是

1. 不是一個很普遍的現象，並
2. 可能相當混亂。

不過，我想我給它一個去，這裏就是我想出了：

用於模擬FOJ最常提到的技術對兩個表是UNION ALL三種可能的情況下， ：

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

或，用真= 1，假= 0

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

這看起來像一個二位整數有三個POSS IBLE非零值，並且SQL提取那些可以是以下形式的

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 

用於與三個表的情況下，因此看起來像一個三比特整數與七個可能的非零值

該狀態表

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 

我想做的加入上一個整數值，所以我只是分配給他們依次對每個七行

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

的，所以我的測試表是

[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 

我創建了一個名爲[foj12]訪問已保存的查詢執行之間的FOJ [表1]和[表2]

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 

它返回

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 

然後，我開始通過涉及案件[表3]工作。它們如下所示，並對應於下面的SQL語句中的「聯合」查詢。

1. 表3同時匹配表1和表2（N = 7）
2. 表3匹配表1但不表2（N = 5）
3. 表3匹配表2，但不表1（N = 6）
4. 行中表3中不表1表2或任何相匹配（N = 4）
5. 行中FOJ（表1，表2）有什麼共同點與表3（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 

個

那小美女回報

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 

（不用說，我沒興趣增加了第四個表！ :)

評論歡迎。