Question

我在数据库中有一个带有主键的表，还有一个“第二个”键。第二个键可以在表中多次出现相同的值，但通常我只想返回该第二个键的最新行。我有一个现有的查询在下面工作，但我觉得它非常难看，应该有一个更简单的方法来做这个而不是创建一个表变量，通过一个循环，并在每次传递时插入表变量循环。我这么做太难了吗？

declare @RowCnt int
declare @MaxRows int
declare @secondID as uniqueidentifier
DECLARE @retList TABLE(
firstGUID  uniqueidentifier,
secondGUID  uniqueidentifier,
name nvarchar(50),
DateCreated  datetime
)

select @RowCnt = 1

declare @Import table (rownum int IDENTITY (1, 1) Primary key NOT NULL , secondGUID uniqueidentifier)
insert into @Import (secondGUID) SELECT DISTINCT dbo.TestTable.secondGUID FROM dbo.TestTable 

select @MaxRows=count(*) from @Import
while @RowCnt <= @MaxRows
begin
    select @secondID=secondGUID from @Import where rownum = @RowCnt 

    INSERT INTO @retList
        SELECT     TOP (1) firstGUID,secondGUID,name,datecreated
        FROM         dbo.TestTable
        WHERE dbo.TestTable.secondGUID = @secondID
        ORDER BY DateCreated Desc

    Set @RowCnt = @RowCnt + 1
END

select * from @retList

编辑：例如，假设表中有这些值

    firstGUID                                secondGUID                              Name     DateCreated
    EAD50999-E9B1-43F0-9FA6-615405FA5A9A     6163B6ED-6AF4-494E-ACE6-184F4804847B    Test1   2014-04-11 15:12:36.303
    A9645486-1021-4E98-92AC-1205CC3FB9D3     6163B6ED-6AF4-494E-ACE6-184F4804847B    Test2  2014-04-10 15:21:46.087
    DEE375BB-BFAF-44BE-AC64-06D7702E2ACB     3BD0A2F0-4E44-43B9-BD24-003B518609C7    Test3
2014-04-11 15:22:37.097

我只希望返回Test1和Test3行。

Answer 1

您可以使用SQLServer的分析功能：

select firstGUID, secondGUID, name, datecreated
from (select t.*, 
             rank() over (partition by secondGUID order by datecreated desc) r
      from TestTable t) ilv
where r=1

Answer 2

我并非100％确定我理解您的要求，但听起来您只想选择包含最大DateCreated的行。这样做的常用方法是使用group by子句的子选择连接，例如：

select tt.*
from TestTable tt
join (
    select firstguid, max(DateCreated) as maxdate
    from TestTable
    group by firstguid
) gtmp on tt.firstguid = gtmp.firstguid and tt.dateCreated = gtmp.maxdate

使用多个键优化SQL查询

2 个答案: