选择具有最大列值的行按另一列选择

Question

This should be a simple question, but I can't get it to work :(

How to select rows that have the maximum column value,as group by another column?

For example,

I have the following table definition:

ID
Del_Index
docgroupviewid

The issue now is that I want to group by results by docgroupviewid first, and then choose one row from each docgroupviewid group, depending on which one has the highest del_index .

I tried

SELECT docgroupviewid, max(del_index),id FROM table
group by docgroupviewid

But instead of return me with the correct id , it returns me with the earliest id from the group with the same docgroupviewid .

Any ideas?

Answer 1

I've struggled with this many times myself and the solution is to think about your query differently.

I want each DocGroupViewID row where the Del_Index is the highest(max) for all rows with that DocGroupViewID:

SELECT
    T.DocGroupViewID,
    T.Del_Index,
    T.ID
FROM MyTable T
WHERE T.Del_Index = (
    SELECT MAX( T1.Del_Index ) FROM MyTable T1
    WHERE T1.DocGroupViewID = T.DocGroupViewID 
)

It gets more complex when more than one row can have the same Del_Index , since then you need some way to choose which one to show.

---

EDIT: wanted to follow up with another option

You can use the RANK() or ROW_NUMBER() functions with a CTE to get more control over the results, as follows:

-- fake a source table
DECLARE @t TABLE (
    ID int IDENTITY(1,1) PRIMARY KEY,
    Del_Index int,
    DocGroupViewID int
)

INSERT INTO @t
SELECT 1, 1 UNION ALL
SELECT 2, 1 UNION ALL
SELECT 3, 1 UNION ALL
SELECT 1, 2 UNION ALL
SELECT 2, 2 UNION ALL
SELECT 2, 2 UNION ALL
SELECT 1, 3 UNION ALL
SELECT 2, 3 UNION ALL
SELECT 3, 3 UNION ALL
SELECT 4, 3

-- show our source
SELECT * FROM @t

-- select using RANK (can have duplicates)
;WITH cteRank AS
(
    SELECT
        DocGroupViewID,
        Del_Index,
        ID,
        RANK() OVER
            (PARTITION BY DocGroupViewID ORDER BY Del_Index DESC)
        AS RowRank,
        ROW_NUMBER() OVER
            (PARTITION BY DocGroupViewID ORDER BY Del_Index DESC)
        AS RowNumber
    FROM @t
)
SELECT *
FROM cteRank
WHERE RowRank = 1

-- select using ROW_NUMBER
;WITH cteRowNumber AS
(
    SELECT
        DocGroupViewID,
        Del_Index,
        ID,
        RANK() OVER
            (PARTITION BY DocGroupViewID ORDER BY Del_Index DESC)
        AS RowRank,
        ROW_NUMBER() OVER
            (PARTITION BY DocGroupViewID ORDER BY Del_Index DESC)
        AS RowNumber
    FROM @t
)
SELECT *
FROM cteRowNumber
WHERE RowNumber = 1

If you have ways to sort out ties, just add it to the ORDER BY .

Answer 2

You will have to complicate your query a little bit:

select a.docgroupviewid, a.del_index, a.id from table a
    where a.del_index = (select max(b.del_index) from table
        where b.docgroupviewid = a.docgroupviewid)