根据一列中的最大值选择分组记录

Question

I cannot get my head around this one, I have a "funcky" subquery:

SELECT  
"Requisition_ID",  
"Candidate_ID",  
"Application_Date",   
CA."Process_of_Activity",  
CA."Activity",   
"Completed_on",  
add_minutes(cast("Completed_on" as timestamp),STAT."Rank") as "Current Completed on"  

FROM CA

left join STATUS as STAT   
on CA."Process_of_Activity" = STAT."Process_of_Activity" and CA."Activity" = STAT."Activity"  
GROUP BY "Requisition_ID","Candidate_ID","Application_Date", STAT."Rank", CA."Process_of_Activity", CA."Activity", "Completed_on";  

This basically gives me a table with 7 columns. I need a result with the same 7 columns GROUPED by the first 3 where the last column is Max. For example. if the original subquery shows me:

Requisition_ID  Candidate_ID    Application_Date    Process_of_Activity Activity         Completed_on   Current Completed on
123              555            16/12/2015          To be Rejected      Rejection        08/03/2016     08/03/2016 00:29
123              555            16/12/2015          To be Rejected      Letter:Rejection 08/03/2016     08/03/2016 00:00
123              555            16/12/2015          Application Entry   Questionnaire    16/12/2015     16/12/2015 00:00
123              555            16/12/2015          Application Entry   Application Entr 08/03/2016     08/03/2016 00:01

in this case I would only want to see the first line of this sample ... so I want to group by "Requisition_ID", "Candidate_ID", "Application_Date"

but also see what the values are for: CA."Process_of_Activity", CA."Activity", "Completed_on"

where "Current Completed on" is max. but of course much more Requisitions, candidates and Application dates in the system. I am using dashDB.

Answer 1

You can use row_number/rank/dense_rank for this purpose. You need something like following:

SELECT *
    FROM 
    (
    SELECT "Requisition_ID",
        "Candidate_ID",
        "Application_Date",
        "Process_of_Activity",
        "Completed_on",
        cco,
        ROW_NUMBER()OVER(PARTITION BY "Requisition_ID",
        "Candidate_ID",
        "Application_Date"
        ORDER BY cco DESC)rn
        FROM 
        (
        SELECT
            "Requisition_ID",
            "Candidate_ID",
            "Application_Date",
            CA."Process_of_Activity",
            CA."Activity",
            "Completed_on",
            add_minutes(CAST("Completed_on" AS TIMESTAMP),STAT."Rank") AS "cco"
            FROM CA
            LEFT JOIN STATUS AS STAT
            ON CA."Process_of_Activity" = STAT."Process_of_Activity" AND CA."Activity" = STAT."Activity"
            GROUP BY "Requisition_ID",
                "Candidate_ID",
                "Application_Date",
                STAT."Rank",
                CA."Process_of_Activity",
                CA."Activity",
                "Completed_on"
        ) AS INNER_QUERY)AS OUTER_QUERY
    WHERE rn=1    

This SQL not compiled on dasdb but row_number function is important. it will give you idea.