how to select top N in access report detail section or select top N in sql subquery

I have a report that is based on a query. In the detail section of the report I want to limit the number of det开发者_开发百科ail lines to top N. (like a select TOP N type query). In this sample I want the top 2 rows. Query I have so far is

 SELECT TestTable.[GroupByCol], TestTable.[DetailColA]
 FROM TestTable group by TestTable.[GroupByCol], TestTable.[DetailColA]

I know I need to join and do some having and count but it's just not clicking. I have also attached a screenshot.

Please advise. thank you

Try the following query as your report recordsource:

SELECT  TestTable.GroupByCol, TestTable.DetailColA
FROM TestTable 
WHERE TestTable.DetailColA IN 
  (SELECT TOP 2 DetailColA FROM TestTable AS TT 
   WHERE TT.GroupByCol=TestTable.GroupByCol 
   ORDER BY TT.DetailColA) 
ORDER BY TestTable.GroupByCol, TestTable.DetailColA

See Allen Browne's Subquery Basics for more information.

EDIT: As you alluded to in your comment, this can also be written as follows:

SELECT a.GroupByCol, a.DetailColA, COUNT(*) As RankNumber
FROM TestTable AS a INNER JOIN TestTable AS b
  ON a.GroupByCol = b.GroupByCol
 AND a.DetailColA >= b.DetailColA
GROUP BY a.GroupByCol, a.DetailColA
HAVING COUNT(*)<=2
ORDER BY a.GroupByCol, a.DetailColA, COUNT(*) 

You would have to benchmark to be sure, but this second version is probably more efficient. Which is more readable is probably a matter of personal preference (I find the first version more intuitive and readable).