从具有公共列的记录集中获取最大值

Question

根据以下记录集，我有三列，需要为每组PersonId返回最大CaseId值。

ID    PersonId  CaseId
66    30        410
1681  30        3508
226   31        958
856   31        2213
1023  31        2400

例如，我想返回一个记录集，例如：

ID    PersonId  CaseId
1681  30        3508
1023  31        2400

如您所见，我总是为所有PersonId集返回最大CaseId。

我尝试了以下内容，但并不总是返回最大记录：

WITH latestRecord AS
 (
  SELECT  CaseId, PersonId, ID,
    ROW_NUMBER() OVER (PARTITION BY PersonId ORDER BY ID ASC) AS RN
  FROM    Employee
  )
  SELECT  Max(RN),CaseId, PersonId
  FROM    latestRecord
  GROUP BY RN,CaseId, PersonId
  --WHERE   RN > 1
  ORDER BY CaseId

感谢您的帮助！

Answer 1

你几乎没错：

WITH latestRecord
AS
(
    SELECT  CaseId, 
            PersonId, 
            ID,
            RN=ROW_NUMBER() OVER (PARTITION BY PersonId ORDER BY CaseId DESC) 
    FROM    Employee
)
SELECT  CaseId, 
        PersonId, 
        ID
FROM    latestRecord
WHERE   RN = 1
ORDER BY CaseId

Answer 2

您还可以使用子查询查找每组PersonId的最大CaseId值。

SELECT *
FROM dbo.Employee t
  JOIN (
        SELECT PersonId, MAX(CaseId) AS CaseId
        FROM dbo.Employee
        GROUP BY PersonId
        ) t2 ON t.PersonId = t2.PersonId AND t.CaseId = t2.CaseId

为了提高性能，请使用以下索引：

CREATE INDEX x ON dbo.Employee(PersonId, CaseId) INCLUDE(ID)

请参阅SQLFiddle

上的演示

Answer 3

这样的事可能吗？

SELECT EMP.ID, DRVDTBL.PERSONID, DRVDTBL.CaseId FROM 
(select  max(PERSONID) as PERSONID, CaseId from Employee
group by CaseId) AS DRVDTBL
INNER JOIN Employee EMP ON DRVDTBL.PERSONID=EMP.PERSONID