Question

我的查询有点像这样：

SELECT ISNULL(MyColumn, "Not Applicable") As MyColumn
FROM MyTable
WHERE SomeOtherColumn =
(SELECT AValue
FROM SomeOtherTable
WHERE SomeConditionHolds
)

如果MyColumn的相关值有一个值，我会得到它，如果它是null，这会让我“不适用”。

但是，偶尔子查询返回空集，在这种情况下整个查询也返回空集。我仍然希望它返回“不适用。”

我能做的最好的就是加入上面的

UNION
SELECT 'Not Applicable' AS MyColumn
WHERE NOT EXISTS
    (SELECT AValue
FROM SomeOtherTable
WHERE SomeConditionHolds
)

但这感觉非常黑客和效率低下。我希望有人有更好的主意。

Answer 1

SELECT ISNULL(MyColumn, DummyCol) As MyColumn
FROM MyTable 
  RIGHT OUTER JOIN (SELECT 'Not Available' AS DummyCol) Q ON DummyCol IS NOT NULL
   AND SomeOtherColumn =
     (SELECT AValue
     FROM SomeOtherTable
     WHERE SomeConditionHolds
     )

Answer 2

试试这个：

    SELECT Case When T.AValue Is Null Then 'Not Applicable' Else MyColumn End As MyColumn
    FROM MyTable Left Outer Join 
   (SELECT AValue
    FROM SomeOtherTable
    WHERE SomeConditionHolds
   ) T on MyTable.SomeOtherColumn  = T.AValue

Answer 3

获取查询结果，当没有Row存在时，将结果设置为“Not Applicable”，否则必须仔细检查并浪费系统时间。

Answer 4

由于您的子查询返回一个值，您可以轻松地将其转换为外部联接，这将始终为您提供一行，这也将使查询更加高效和优雅：

SELECT
    ISNULL(MyColumn, "Not Applicable") As MyColumn
FROM MyTable
LEFT JOIN SomeOtherTable
    ON SomeOtherColumn = AValue
    AND SomeConditionHolds

这里的关键是SomeConditionHolds在 join 条件中，不是 where子句，否则左连接有效地成为内连接。< / p>

Answer 5

declare @Foo as Table ( FooId Int Identity, Something VarChar(16) );
insert into @Foo ( Something ) values
  ( 'One' ), ( 'II' ), ( '3' );

declare @FooLimit as Int = 1; -- Try 5.

with Alicia as (
  select FooId, Something, 1 as SetId
    from @Foo
    where FooId >= @FooLimit
  union all
  select 42, 'n/a', 2 )
  select FooId, Something
    from Alicia
    where SetId = ( select Min( SetId ) from Alicia )

返回一个值，即使where中的子查询返回空集

5 个答案: