外连接中出现意外的 NULL
Unexpected NULLs in outer join
我有 3 个表要加入。我们称它们为 TableA、TableB 和 TableC：
DECLARE @TableA TABLE
(
Key1 int,
PRIMARY KEY
(
Key1
)
)
DECLARE @TableB TABLE
(
Key1 int,
Key2 int,
PRIMARY KEY
(
Key1,
Key2
)
)
DECLARE @TableC TABLE
(
Key3 int NOT NULL,
Key1 int NOT NULL,
Key2 int NULL,
PRIMARY KEY
(
Key3
)
)
下面是一些示例数据：
INSERT INTO @TableA (Key1) VALUES (1);
INSERT INTO @TableB (Key1, Key2) VALUES (1, 1), (1, 2), (1, 3), (1, 4)
INSERT INTO @TableC (Key3, Key1, Key2) VALUES (1, 1, NULL), (2, 1, NULL), (3, 1, 1), (4, 1, 3)
TableB 和 TableC 都有通过 Key1 到 TableA 的外键。实际上，如果 Key2 不为 null，则 TableC 也可以通过 Key1 和 Key2 组合来引用 TableB，但没有实际的外键。 Key3 无关紧要，除了 Key1 和 Key2 不是 TableC 的主键的一部分。
我正在尝试编写结合了 TableB 和 TableC 的查询：
SELECT
TableA.Key1 AS [A1],
TableB.Key1 AS [B1],
TableB.Key2 AS [B2],
TableC.Key1 AS [C1],
TableC.Key2 AS [C2],
TableC.Key3 AS [C3]
FROM @TableA AS TableA
FULL OUTER JOIN @TableC AS TableC
ON TableC.Key1 = TableA.Key1
FULL OUTER JOIN @TableB AS TableB
ON (TableB.Key1 = TableA.Key1 AND TableC.Key1 IS NULL)
OR (TableC.Key1 = TableB.Key1 AND TableC.Key2 = TableB.Key2)
WHERE (TableA.Key1 = TableB.Key1 OR TableA.Key1 = TableC.Key1)
ORDER BY TableB.Key2, TableC.Key2
我的期望是 TableB 和 TableC 都应该包含它们的所有行，匹配在两个键上匹配的行，以及不匹配的 NULLS。
我希望得到这个：
A1 B1 B2 C1 C2 C3
1 NULL NULL 1 NULL 1
1 NULL NULL 1 NULL 2
1 1 1 1 1 3
1 1 2 NULL NULL NULL -- THIS ROW IS MISSING
1 1 3 1 3 4
1 1 4 NULL NULL NULL -- THIS ROW IS MISSING
但是我得到的是：
A1 B1 B2 C1 C2 C3
1 NULL NULL 1 NULL 1
1 NULL NULL 1 NULL 2
1 1 1 1 1 3
1 1 3 1 3 4
如果我注释掉 WHERE 子句，我会得到我期望的所有行，除了 A1 对于缺失的行为 NULL：
A1 B1 B2 C1 C2 C3
1 NULL NULL 1 NULL 1
1 NULL NULL 1 NULL 2
1 1 1 1 1 3
NULL 1 2 NULL NULL NULL -- A1 should be 1
1 1 3 1 3 4
NULL 1 4 NULL NULL NULL -- A1 should be 1
为什么 TableA.Key1 返回 NULL 并导致它排除缺少 TableB.Key2 的行？
编辑：
这是我了解自己做错了什么之后的最终固定查询：
SELECT
TableA.Key1 AS A1,
Subquery.*
FROM @TableA AS TableA
INNER JOIN
(
SELECT
TableB.Key1 AS [B1],
TableB.Key2 AS [B2],
TableC.Key1 AS [C1],
TableC.Key2 AS [C2],
TableC.Key3 AS [C3]
FROM @TableC AS TableC
FULL OUTER JOIN @TableB AS TableB
ON TableB.Key1 = TableC.Key1 AND TableB.Key2 = TableC.Key2
) AS Subquery
ON Subquery.B1 = TableA.Key1 OR Subquery.C1 = TableA.Key1
ORDER BY Subquery.B2, Subquery.C2
Why is TableA.Key1 coming back NULL and causing it to exclude rows
where TableB.Key2 is missing?
完整外部联接与 INNER JOIN 相同，但任何一侧的任何不匹配的行都将添加回另一侧的列 NULL。
您的查询首先对 A 和 C 进行完全外部联接，因此请先查看其结果。
SELECT
TableA.Key1 AS [A1],
TableC.Key1 AS [C1],
TableC.Key2 AS [C2],
TableC.Key3 AS [C3]
FROM @TableA AS TableA
FULL OUTER JOIN @TableC AS TableC
ON TableC.Key1 = TableA.Key1
这个returns下面的虚拟table（VT1）进入下一阶段。由于这与 INNER JOIN 的结果相同，我怀疑它是否需要任何解释。 @TableC 中的每一行都成功匹配了 @TableA 中的单行。
+----+----+------+----+
| A1 | C1 | C2 | C3 |
+----+----+------+----+
| 1 | 1 | NULL | 1 |
| 1 | 1 | NULL | 2 |
| 1 | 1 | 1 | 3 |
| 1 | 1 | 3 | 4 |
+----+----+------+----+
然后将其完全外部连接到 B。 B的内容是
+------+------+
| Key1 | Key2 |
+------+------+
| 1 | 1 |
| 1 | 2 |
| 1 | 3 |
| 1 | 4 |
+------+------+
INNER JOIN那两个带谓词ON (TableB.Key1 = [A1] AND [C1] IS NULL) OR ([C1] = TableB.Key1 AND [C2] = TableB.Key2)returns的结果集只有2行。
+----+----+----+----+----+----+
| A1 | B1 | B2 | C1 | C2 | C3 |
+----+----+----+----+----+----+
| 1 | 1 | 1 | 1 | 1 | 3 |
| 1 | 1 | 3 | 1 | 3 | 4 |
+----+----+----+----+----+----+
来自 VT1 的不匹配行按照 LEFT JOIN 添加回（这些是 C3 是 1 或 2 的行）
+----+------+------+----+------+----+
| A1 | B1 | B2 | C1 | C2 | C3 |
+----+------+------+----+------+----+
| 1 | NULL | NULL | 1 | NULL | 1 |
| 1 | NULL | NULL | 1 | NULL | 2 |
| 1 | 1 | 1 | 1 | 1 | 3 |
| 1 | 1 | 3 | 1 | 3 | 4 |
+----+------+------+----+------+----+
和来自 B 的不匹配行，根据 RIGHT JOIN（这些是 B2 是 2 或 4 的行）
给你最终结果
+------+------+------+------+------+------+
| A1 | B1 | B2 | C1 | C2 | C3 |
+------+------+------+------+------+------+
| 1 | NULL | NULL | 1 | NULL | 1 |
| 1 | NULL | NULL | 1 | NULL | 2 |
| 1 | 1 | 1 | 1 | 1 | 3 |
| 1 | 1 | 3 | 1 | 3 | 4 |
| NULL | 1 | 2 | NULL | NULL | NULL |
| NULL | 1 | 4 | NULL | NULL | NULL |
+------+------+------+------+------+------+
这就是您想要的——注意...您想要 B 和 C 上的完整外部，因此 A 无关紧要——在您的示例的查询中甚至不需要它，但您可以离开或根据需要进行内部连接（我使用左连接）
SELECT
TableA.Key1 AS [A1], -- Probably not needed
TableB.Key1 AS [B1],
TableB.Key2 AS [B2],
TableC.Key1 AS [C1],
TableC.Key2 AS [C2],
TableC.Key3 AS [C3]
FROM @TableB AS TableB
FULL OUTER JOIN @TableC AS TableC ON TableB.Key1 = TableC.Key1 and TableB.Key2 = TableC.Key2
LEFT JOIN @TableA AS TableA ON TableB.Key1 = TableA.Key1 -- Probably not needed
SELECT
a.Key1 AS [A1],
b.Key1 AS [B1],
b.Key2 AS [B2],
c.Key1 AS [C1],
c.Key2 AS [C2],
c.Key3 AS [C3]
FROM @TableB b
LEFT JOIN @TableC c
ON c.Key2 = b.Key2
INNER JOIN @TableA a
ON b.Key1 = a.Key1
UNION
SELECT
a.Key1 AS [A1],
b.Key1 AS [B1],
b.Key2 AS [B2],
c.Key1 AS [C1],
c.Key2 AS [C2],
c.Key3 AS [C3]
FROM @TableC c
LEFT JOIN @TableB b
ON c.Key2 = b.Key2
INNER JOIN @TableA a
ON c.Key1 = a.Key1
输出：
A1 B1 B2 C1 C2 C3
1 NULL NULL 1 NULL 1
1 NULL NULL 1 NULL 2
1 1 1 1 1 3
1 1 2 NULL NULL NULL
1 1 3 1 3 4
1 1 4 NULL NULL NULL
我先拿到B面，再拿到C面，用union把他们拉在一起
希望这对你有帮助...
我有 3 个表要加入。我们称它们为 TableA、TableB 和 TableC：
DECLARE @TableA TABLE
(
Key1 int,
PRIMARY KEY
(
Key1
)
)
DECLARE @TableB TABLE
(
Key1 int,
Key2 int,
PRIMARY KEY
(
Key1,
Key2
)
)
DECLARE @TableC TABLE
(
Key3 int NOT NULL,
Key1 int NOT NULL,
Key2 int NULL,
PRIMARY KEY
(
Key3
)
)
下面是一些示例数据：
INSERT INTO @TableA (Key1) VALUES (1);
INSERT INTO @TableB (Key1, Key2) VALUES (1, 1), (1, 2), (1, 3), (1, 4)
INSERT INTO @TableC (Key3, Key1, Key2) VALUES (1, 1, NULL), (2, 1, NULL), (3, 1, 1), (4, 1, 3)
TableB 和 TableC 都有通过 Key1 到 TableA 的外键。实际上，如果 Key2 不为 null，则 TableC 也可以通过 Key1 和 Key2 组合来引用 TableB，但没有实际的外键。 Key3 无关紧要，除了 Key1 和 Key2 不是 TableC 的主键的一部分。
我正在尝试编写结合了 TableB 和 TableC 的查询：
SELECT
TableA.Key1 AS [A1],
TableB.Key1 AS [B1],
TableB.Key2 AS [B2],
TableC.Key1 AS [C1],
TableC.Key2 AS [C2],
TableC.Key3 AS [C3]
FROM @TableA AS TableA
FULL OUTER JOIN @TableC AS TableC
ON TableC.Key1 = TableA.Key1
FULL OUTER JOIN @TableB AS TableB
ON (TableB.Key1 = TableA.Key1 AND TableC.Key1 IS NULL)
OR (TableC.Key1 = TableB.Key1 AND TableC.Key2 = TableB.Key2)
WHERE (TableA.Key1 = TableB.Key1 OR TableA.Key1 = TableC.Key1)
ORDER BY TableB.Key2, TableC.Key2
我的期望是 TableB 和 TableC 都应该包含它们的所有行，匹配在两个键上匹配的行，以及不匹配的 NULLS。
我希望得到这个：
A1 B1 B2 C1 C2 C3
1 NULL NULL 1 NULL 1
1 NULL NULL 1 NULL 2
1 1 1 1 1 3
1 1 2 NULL NULL NULL -- THIS ROW IS MISSING
1 1 3 1 3 4
1 1 4 NULL NULL NULL -- THIS ROW IS MISSING
但是我得到的是：
A1 B1 B2 C1 C2 C3
1 NULL NULL 1 NULL 1
1 NULL NULL 1 NULL 2
1 1 1 1 1 3
1 1 3 1 3 4
如果我注释掉 WHERE 子句，我会得到我期望的所有行，除了 A1 对于缺失的行为 NULL：
A1 B1 B2 C1 C2 C3
1 NULL NULL 1 NULL 1
1 NULL NULL 1 NULL 2
1 1 1 1 1 3
NULL 1 2 NULL NULL NULL -- A1 should be 1
1 1 3 1 3 4
NULL 1 4 NULL NULL NULL -- A1 should be 1
为什么 TableA.Key1 返回 NULL 并导致它排除缺少 TableB.Key2 的行？
编辑：
这是我了解自己做错了什么之后的最终固定查询：
SELECT
TableA.Key1 AS A1,
Subquery.*
FROM @TableA AS TableA
INNER JOIN
(
SELECT
TableB.Key1 AS [B1],
TableB.Key2 AS [B2],
TableC.Key1 AS [C1],
TableC.Key2 AS [C2],
TableC.Key3 AS [C3]
FROM @TableC AS TableC
FULL OUTER JOIN @TableB AS TableB
ON TableB.Key1 = TableC.Key1 AND TableB.Key2 = TableC.Key2
) AS Subquery
ON Subquery.B1 = TableA.Key1 OR Subquery.C1 = TableA.Key1
ORDER BY Subquery.B2, Subquery.C2
Why is TableA.Key1 coming back NULL and causing it to exclude rows where TableB.Key2 is missing?
完整外部联接与 INNER JOIN 相同，但任何一侧的任何不匹配的行都将添加回另一侧的列 NULL。
您的查询首先对 A 和 C 进行完全外部联接，因此请先查看其结果。
SELECT
TableA.Key1 AS [A1],
TableC.Key1 AS [C1],
TableC.Key2 AS [C2],
TableC.Key3 AS [C3]
FROM @TableA AS TableA
FULL OUTER JOIN @TableC AS TableC
ON TableC.Key1 = TableA.Key1
这个returns下面的虚拟table（VT1）进入下一阶段。由于这与 INNER JOIN 的结果相同，我怀疑它是否需要任何解释。 @TableC 中的每一行都成功匹配了 @TableA 中的单行。
+----+----+------+----+
| A1 | C1 | C2 | C3 |
+----+----+------+----+
| 1 | 1 | NULL | 1 |
| 1 | 1 | NULL | 2 |
| 1 | 1 | 1 | 3 |
| 1 | 1 | 3 | 4 |
+----+----+------+----+
然后将其完全外部连接到 B。 B的内容是
+------+------+
| Key1 | Key2 |
+------+------+
| 1 | 1 |
| 1 | 2 |
| 1 | 3 |
| 1 | 4 |
+------+------+
INNER JOIN那两个带谓词ON (TableB.Key1 = [A1] AND [C1] IS NULL) OR ([C1] = TableB.Key1 AND [C2] = TableB.Key2)returns的结果集只有2行。
+----+----+----+----+----+----+
| A1 | B1 | B2 | C1 | C2 | C3 |
+----+----+----+----+----+----+
| 1 | 1 | 1 | 1 | 1 | 3 |
| 1 | 1 | 3 | 1 | 3 | 4 |
+----+----+----+----+----+----+
来自 VT1 的不匹配行按照 LEFT JOIN 添加回（这些是 C3 是 1 或 2 的行）
+----+------+------+----+------+----+
| A1 | B1 | B2 | C1 | C2 | C3 |
+----+------+------+----+------+----+
| 1 | NULL | NULL | 1 | NULL | 1 |
| 1 | NULL | NULL | 1 | NULL | 2 |
| 1 | 1 | 1 | 1 | 1 | 3 |
| 1 | 1 | 3 | 1 | 3 | 4 |
+----+------+------+----+------+----+
和来自 B 的不匹配行，根据 RIGHT JOIN（这些是 B2 是 2 或 4 的行）
给你最终结果
+------+------+------+------+------+------+
| A1 | B1 | B2 | C1 | C2 | C3 |
+------+------+------+------+------+------+
| 1 | NULL | NULL | 1 | NULL | 1 |
| 1 | NULL | NULL | 1 | NULL | 2 |
| 1 | 1 | 1 | 1 | 1 | 3 |
| 1 | 1 | 3 | 1 | 3 | 4 |
| NULL | 1 | 2 | NULL | NULL | NULL |
| NULL | 1 | 4 | NULL | NULL | NULL |
+------+------+------+------+------+------+
这就是您想要的——注意...您想要 B 和 C 上的完整外部，因此 A 无关紧要——在您的示例的查询中甚至不需要它，但您可以离开或根据需要进行内部连接（我使用左连接）
SELECT
TableA.Key1 AS [A1], -- Probably not needed
TableB.Key1 AS [B1],
TableB.Key2 AS [B2],
TableC.Key1 AS [C1],
TableC.Key2 AS [C2],
TableC.Key3 AS [C3]
FROM @TableB AS TableB
FULL OUTER JOIN @TableC AS TableC ON TableB.Key1 = TableC.Key1 and TableB.Key2 = TableC.Key2
LEFT JOIN @TableA AS TableA ON TableB.Key1 = TableA.Key1 -- Probably not needed
SELECT
a.Key1 AS [A1],
b.Key1 AS [B1],
b.Key2 AS [B2],
c.Key1 AS [C1],
c.Key2 AS [C2],
c.Key3 AS [C3]
FROM @TableB b
LEFT JOIN @TableC c
ON c.Key2 = b.Key2
INNER JOIN @TableA a
ON b.Key1 = a.Key1
UNION
SELECT
a.Key1 AS [A1],
b.Key1 AS [B1],
b.Key2 AS [B2],
c.Key1 AS [C1],
c.Key2 AS [C2],
c.Key3 AS [C3]
FROM @TableC c
LEFT JOIN @TableB b
ON c.Key2 = b.Key2
INNER JOIN @TableA a
ON c.Key1 = a.Key1
输出：
A1 B1 B2 C1 C2 C3
1 NULL NULL 1 NULL 1
1 NULL NULL 1 NULL 2
1 1 1 1 1 3
1 1 2 NULL NULL NULL
1 1 3 1 3 4
1 1 4 NULL NULL NULL
我先拿到B面，再拿到C面，用union把他们拉在一起
希望这对你有帮助...