为什么T-SQL排名值会返回可为空的列？

Question

使用T-SQL的排名函数时,包含排名值的列可以为空.

您可以在创建结果集视图时看到此信息:

CREATE VIEW v
AS 
  SELECT Name
    , ListPrice
    , RANK() OVER (ORDER BY ListPrice DESC) AS [Rank]
    , DENSE_RANK() OVER (ORDER BY ListPrice DESC) AS [DenseRank]
    , ROW_NUMBER() OVER (ORDER BY ListPrice DESC) AS [RowNumber]
FROM Production.Product

执行sp_help此视图表示使用排名函数的列可以为空:

EXEC sp_help 'v'

Column_name   (...) | Nullable
---------------...-+------------+
...           (...) | ...
Rank          (...) | Yes
DenseRank     (...) | Yes
RowNumber     (...) | Yes

哪种情况会导致排名函数返回NULL？

Answer 1

视图中的每个基于计算/函数的列似乎都可以为空.例如:

create view v1
as
    select OBJECT_ID,OBJECT_ID * 1 as obj2 from sys.objects
go
EXEC sp_help 'v1'

表示object_id不可为空,但obj2即使它是object_id可以观察到的,如果永远不能为null,也可以obj2.

我知道的唯一方法(不确定这是否是你真正想要的)强制列看起来不可为空,是将它包装在ISNULL:

create view v2
as
    select OBJECT_ID,ISNULL(OBJECT_ID * 1,0) as obj2 from sys.objects
go
EXEC sp_help 'v2'

有趣的是,这是您无法使用的少数几个地方之一,COALESCE而不是ISNULL:

create view v3
as
    select OBJECT_ID,COALESCE(OBJECT_ID * 1,0) as obj2 from sys.objects
go
EXEC sp_help 'v3'

v3类似的v1.