当任一表可能返回零结果时，SQL 从多个表返回一个结果集

Question

我有两个表，其中有些列相似，有些列不同。我需要返回一个将不同列合并到一个结果集中的结果，但是，我需要一个可能导致两个表中的任何一个都不匹配的条件，我尝试了一个联合，但它返回了具有空值的两行，并且我只想要一个。下面是两个示例表:

TableA
----------------------------------------------------
| ID | ColumnA | ColumnB | ForeignKeyA | TimeStamp |
----------------------------------------------------
| 1  | Val1    | Val2    | KeyA        | 2013-01-01|
----------------------------------------------------
| 2  | Val3    | Val4    | KeyB        | 2013-01-02|
----------------------------------------------------

TableB
------------------------------------------
| ID | ColumnC | ForeignKeyA | TimeStamp |
------------------------------------------
| 1  | Val5    | KeyA        | 2013-01-01|
------------------------------------------
| 2  | Val6    | KeyC        | 2013-01-02|
------------------------------------------

这里有一些伪查询和我想要的返回值:

1)

SELECT TableA.ColumnA AS ColumnA, 
TableA.ColumnB AS Column B, 
TableB.ColumnC  AS ColumnC,
TableA.id AS TableA_ID,
TableB.id AS TableB_ID
(WHERE ForeignKeyA in either table = KeyA and TimeStamp in either table = 2013-01-01)

>>
-------------------------------------------------------
| ColumnA | ColumnB | ColumnC | TableA_ID | TableB_ID |
-------------------------------------------------------
| Val1    | Val2    | Val5    | 1         | 1         |
-------------------------------------------------------

2)

SELECT TableA.ColumnA AS ColumnA, 
TableA.ColumnB AS Column B, 
TableB.ColumnC  AS ColumnC,
TableA.id AS TableA_ID,
TableB.id AS TableB_ID
(WHERE ForeignKeyA in either table = KeyB and TimeStamp in either table = 2013-01-02)

>>
 -------------------------------------------------------
| ColumnA | ColumnB | ColumnC | TableA_ID | TableB_ID |
-------------------------------------------------------
| Val3    | Val4    | Null    | 2         | Null      |
-------------------------------------------------------

3)

SELECT TableA.ColumnA AS ColumnA, 
TableA.ColumnB AS Column B, 
TableB.ColumnC  AS ColumnC,
TableA.id AS TableA_ID,
TableB.id AS TableB_ID
(WHERE ForeignKeyA in either table = KeyC and TimeStamp in either table = 2013-01-02)

>>
 -------------------------------------------------------
| ColumnA | ColumnB | ColumnC | TableA_ID | TableB_ID |
-------------------------------------------------------
| Null    | Null    | Val6    | Null      | 2         |
-------------------------------------------------------

Answer 1

我认为在您的查询中您可以像这样使用完全外部联接(您可以将列添加到on 子句中，但它不会改变任何内容):

with
cteA as (select * from TableA where TimeStamp = _ts and ForeignKeyA = _fk),
cteB as (select * from TableB where TimeStamp = _ts and ForeignKeyA = _fk)
select
    A.ColumnA, A.ColumnB, B.ColumnC,
    A.ID as TableA_ID, B.ID as TableB_ID
from cteA as A
    full outer join cteB as B on 1 = 1

但是您可以在不进行预过滤的情况下使用完全外部联接(如果您在ForeignKeyA 和TimeStamp 列上有索引，这会降低效率):

select
    A.ColumnA, A.ColumnB, B.ColumnC,
    A.ID as TableA_ID, B.ID as TableB_ID
from TableA as A
    full outer join TableB as B
        on B.ForeignKeyA = A.ForeignKeyA and B.TimeStamp = A.TimeStamp
where
    coalesce(A.ForeignKeyA, B.ForeignKeyA) = _fk and
    coalesce(A.TimeStamp, B.TimeStamp) = _ts

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