我学到了 Access 不允许您进行完全外部联接的艰难方式，并且在阅读如何模拟外部联接时，我已经开始了解如何做到这一点，但在尝试将其应用于不止只有两张桌子。
就这么简单吗？
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
【问题讨论】：
快速搜索网络发现，模拟 FULL OUTER JOIN（“FOJ”）的主题已经在 Stack Overflow 和其他地方讨论过很多次，但答案似乎都仅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
不过，我想我会试一试，这就是我想出的：
在两个表上模拟 FOJ 最常提到的技术是将所有三种可能的情况联合起来：
In_Table2 In_Table1
--------- ---------
false true
true false
true true
或者，替换为 true=1 和 false=0
In_Table2 In_Table1
--------- ---------
0 1
1 0
1 1
这看起来像一个具有三个可能的非零值的两位整数，提取这些值的 SQL 格式为
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
因此，具有三个表的情况的状态表看起来像一个具有七个可能的非零值的三位整数
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
我想对一个整数值进行连接，所以我只是为七行中的每一行依次分配它们
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] 的已保存查询来执行 [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
然后我开始处理涉及 [Table3] 的案例。它们如下所示，对应于下面 SQL 语句中的“联合”查询。
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
那个小美女回来了
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
（不用说我对添加第四个表不感兴趣！:)
欢迎评论。
【讨论】：