SQL使用OVER和PARTITION BY

Question

I have the following table;

ClientID    | Location  | Episode   | Date  
001         | Area1     | 4         | 01Dec16  
001         | Area2     | 3         | 01Nov16  
001         | Area2     | 2         | 01Oct16  
001         | Area1     | 1         | 01Sep16  
002         | Area2     | 3         | 21Dec16  
002         | Area1     | 2         | 21Nov16  
002         | Area1     | 1         | 21Oct16    

And I'm looking to create 2 new columns based to the latest episode of the client

ClientID    | Location  | Episode   | Date  | LatestEpisode     | LatestLocation   
001         | Area1     | 4         | Dec   | 4                 | Area1  
001         | Area2     | 3         | Nov   | 4                 | Area1   
001         | Area2     | 2         | Oct   | 4                 | Area1  
001         | Area1     | 1         | Sep   | 4                 | Area1  
002         | Area2     | 3         | Dec   | 3                 | Area2  
002         | Area1     | 2         | Nov   | 3                 | Area2  
002         | Area1     | 1         | Oct   | 3                 | Area2      

I have worked out I can use OVER to work out the LatestEspisode: LatestEpisode = MAX(Episode) OVER(PARTITION BY ClientID)

But can't work out how to get the LatestLocation?

EDIT: Sorry if I haven't got the format right, this is my first post. I was trying to look at how to post correctly but I found it quite confusing

I have searched stackoverflow many times over the last 3 days and have found various ways using OVER and ROW NUMBER() but I don't have a lot of experience of them. Many of the examples I had found previously were fine for producing an aggregated table but I want to keep the full table, this is why I thought using OVER was the way to go.

Answer 1

Sql server 2012 version introduced the FIRST_VALUE() function, That enables you to write your select query like this:

SELECT  ClientID, 
        Location, 
        Episode, 
        [Date], 
        LatestEpisode = FIRST_VALUE(Episode) OVER(PARTITION BY ClientID ORDER BY [Date] DESC), 
        LatestLocation = FIRST_VALUE(Location) OVER(PARTITION BY ClientID ORDER BY [Date] DESC) 
FROM tableName

Answer 2

In SQL Server, I would do this with cross apply :

select e.*, e2.episode as LatestEpisode, e2.location as LatestLocation
from episodes e cross apply
     (select top 1 e2.*
      from episodes e2
      where e2.clientId = e.clientId
      order by e2.episode desc
     ) elast;

Although you can express this logic with window functions, the lateral join (implemented in SQL Server using the apply keyword) is more natural way of expressing the logic.

If you are not familiar with lateral joins, you can think of them as a correlated subqueries in the from clause . . . but queries that allow you to return multiple columns. I should add, though, that one of the main use cases is for table-valued functions, so it is a very powerful construct.

Answer 3

First, you need to select LatestEpisode per each client and then you can use this value to identify row, where you can get LatestLocation from

SELECT *
    ,(
        SELECT Location
        FROM Episodes
        WHERE ClientId = MyTable.ClientId
            AND Episode = MyTable.LatestEpisode
        ) AS LatestLocation
FROM (
    SELECT *
        ,MAX(Episode) OVER (PARTITION BY ClientId) AS LatestEpisode
    FROM Episodes
    ) AS MyTable

You can also use common table expression (CTE):

WITH cte
AS (
    SELECT *
        ,MAX(Episode) OVER (PARTITION BY ClientId) AS LatestEpisode
    FROM Episodes
    )
SELECT cte.*
    ,(
        SELECT Location
        FROM Episodes
        WHERE ClientId = cte.ClientId
            AND Episode = cte.LatestEpisode
        ) AS LatestLocation
FROM cte

Answer 4

I have worked on it and able to produce the required result Please try below

Declare @Table table ( ClientID varchar(max), Location varchar(500), Episode int, Dated varchar(30)) 

Insert Into @Table 
Values ('001', 'Area1', 4 ,'01Dec16' )
,('001', 'Area2', 3, '01Nov16')
, ('001', 'Area2', 2, '01Oct16')  
,('001' ,'Area1' ,1, '01Sep16')
,('002' ,'Area2' ,3, '21Dec16') 
,('002' ,'Area1' ,2, '21Nov16') 
,('002' ,'Area1' ,1, '21Oct16') 


; WITH LL AS 
(
SELECT CLientID ,MAX(CAST (Dated as Date)) as maxdate
FROM @table 
GROUP BY ClientID
) 
, Area AS 
(
SELECT Location, x.ClientID, x.Dated FROM @Table x INNER JOIN LL b ON x.ClientID = b.ClientID AND x.Dated = b.maxdate
) 
SELECT a.*
, LatestEpisode = MAX(Episode) OVER(PARTITION BY a.ClientID)
, LatestLocation =  b.Location
FROM @Table a 
INNER JOIN Area b ON a.ClientID = b.ClientID