SQL：过滤（行）在组（列）中具有最大值

Question

I need to filter for rows based on maximum values of version within month and location. Using SQL.

For example, I have table below where there are version 1 and 2 of June & NYC, I wanted to filter for only the row of version 2 with revenue 11. Or for January & NYC, I wanted to get only the row with revenue 15.

Month   Location Version    Revenue
June    NYC     1            10
June    NYC     2            11
June    LA      3            12
January NYC     1            13
January NYC     2            14
January NYC     3            15
January LA      1            16
January LA      2            17

Result:

Month   Location  Version   Revenue
June    NYC        2          11
June    LA         3          12
January NYC        3          15
January LA         2          17

Edit to change name of column to Revenue to remove confusion. I do not need the max value of revenue, only revenue that goes with max version of that month and that location.

Answer 1

You can also use joins as an alternative to correlated subqueries, eg:

select t1.* from YourTable t1 inner join
(
    select t2.month, t2.location, max(t2.version) as mv
    from YourTable t2
    group by t2.month, t2.location
) q on t1.month = q.month and t1.location = q.location and t1.version = q.mv

Change YourTable to the name of your table.

Answer 2

A typical method is filtering using a correlated subquery:

select t.*
from t
where t.version = (select max(t2.version)
                   from t t2
                   where t2.month = t.month and t2.location = t.location
                  );

Answer 3

Another alternative that minimizes subqueries is to use the row_number() window function. (You don't mention which database server you're using, but most of them support it.)

SELECT month, location, version, revenue
FROM (SELECT month, location, version, revenue
           , row_number() OVER (PARTITION BY month, location ORDER BY version DESC) AS rn
      FROM your_table)
WHERE rn = 1;