How to find the record in a table that contains the maximum value?

Question

Although this question looks simple, it is kind of tricky.

I have a table with the following columns:

table A:
  int ID
  float value
  datetime date         


        

Answer 1

If date is unique, then you already have your answer. If date is not unique, then you need some other uniqueifier. Absent a natural key, your ID is as good as any. Just put a MAX (or MIN, whichever you prefer) on it:

SELECT *
FROM A
JOIN (
    --Dedupe any non unqiue dates by getting the max id for each group that has the max date
    SELECT Group, MAX(Id) as Id
    FROM A 
    JOIN (
        --Get max date for each group
        SELECT group, MAX(date) as Date 
        FROM A 
        GROUP BY group
    ) as MaxDate ON
        A.Group = MaxDate.Group
        AND A.Date = MaxDate.Date
    GROUP BY Group
) as MaxId ON
    A.Group = MaxId.Group
    AND A.Id= MaxId.Id