sql query - how to apply limit within group by

Question

I have a table named t1 with following fields: ROWID, CID, PID, Score, SortKey

it has the following data:

1, C1, P1, 10, 1
2, C1, P2, 20, 2
3, C1, P3         


        

Answer 1

This is not actually a GROUP BY problem (you're not aggregating values). This is a greatest-n-per-group problem (I think there's actually a greatest-n-per-group tag here at Stackoverflow).

The exact details of a solution will depend on issues such as whether you ever have the same sort key twice per group. You can start with something like this:

SELECT * FROM table T1 WHERE Score > 20 AND
  (SELECT COUNT(*) FROM table T2 
      WHERE T2.CID = T1.CID AND T2.SortKey <= T1.SortKey AND T2.RowID <> T1.RowID
        AND T1.Score > 20) < 2;
  ORDER BY CID, SortKey;

What this does is consider only those rows with scores above 20. Then, for each candidate row it counts the number of other rows in the same table that have scores > 20 but sortkeys less than or equal to this row's sortkey. If that number is 0 or 1 row, then this row qualifies for inclusion in the results.

Finally ORDER by performs your sort.