select top n record from each group sqlite - sql

Question

I am trying to select top 2 records from a database table result that looks like this
SubjectId | StudentId | Levelid | total
------------------------------------------
1 | 1 | 1 | 89
1 | 2 | 1 | 77
1 | 3 | 1 | 61
2 | 4 | 1 | 60
2 | 5 | 1 | 55
2 | 6 | 1 | 45
i tried this query
SELECT rv.subjectid,
rv.total,
rv.Studentid,
rv.levelid
FROM ResultView rv
LEFT JOIN ResultView rv2
ON ( rv.subjectid = rv2.subjectid
AND
rv.total <= rv2.total )
GROUP BY rv.subjectid,
rv.total,
rv.Studentid
HAVING COUNT( * ) <= 2
order by rv.subjectid desc
but some subjects like where missing, i even tried the suggestiong frm the following link
How to select the first N rows of each group?
but i get more that two for each subjectid
what am i doing wrong?

Answer 1

You could use a correlated subquery:
select *
from ResultView rv1
where SubjectId || '-' || StudentId || '-' || LevelId in
(
select SubjectId || '-' || StudentId || '-' || LevelId
from ResultView rv2
where SubjectID = rv1.SubjectID
order by
total desc
limit 2
)
This query constructs a single-column primary key by concatenating three columns. If you have a real primary key (like ResultViewID) you can substitute that for SubjectId || '-' || StudentId || '-' || LevelId.
Example at SQL Fiddle.

Answer 2

I hope I'm understanding your question correctly. Let me know if this is correct:
I recreated your table:
CREATE TABLE stack (
SubjectId INTEGER(10),
StudentId INTEGER(10),
Levelid INTEGER(10),
total INTEGER(10)
)
;
Inserted values
INSERT INTO stack VALUES
(1,1,1,89),
(1,2,1,77),
(1,3,1,61),
(2,4,1,60),
(2,5,1,55),
(2,6,1,45)
;
If you're trying to get the top group by Levelid (orderd by total field, assuming StudentID as primary key):
SELECT *
FROM stack AS a
WHERE a.StudentID IN (
SELECT b.StudentID
FROM stack AS b
WHERE a.levelid = b.levelid
ORDER BY b.total DESC
LIMIT 2
)
;
Yields this result:
SubjectId | StudentId | Levelid | total
1 | 1 | 1 | 89
1 | 2 | 1 | 77
Example of top 2 by SubjectId, ordered by total:
SELECT *
FROM stack AS a
WHERE a.StudentID IN (
SELECT b.StudentID
FROM stack AS b
WHERE a.subjectID = b.subjectID
ORDER BY b.total DESC
LIMIT 2
)
;
Result:
SubjectId | StudentId | Levelid | total
1 | 1 | 1 | 89
1 | 2 | 1 | 77
2 | 4 | 1 | 60
2 | 5 | 1 | 55
I hope that was the answer you were looking for.

Answer 3

ROW_NUMBER window function
SQLite now supports window functions, so the exact same code that works for PostgreSQL at Grouped LIMIT in PostgreSQL: show the first N rows for each group? now also works for SQLite.
This could be potentially faster than the other answers so far as it does not run a correlated subquery.
Supposing you want to get the 2 highest total rows for each StudentID:
SELECT *
FROM (
SELECT
ROW_NUMBER() OVER (
PARTITION BY "StudentID"
ORDER BY "total" DESC
) AS "rnk",
*
FROM "mytable"
) sub
WHERE
"sub"."rnk" <= 2
ORDER BY
"sub"."StudentID" ASC,
"sub"."total" DESC
Tested on SQLite 3.34, PostgreSQL 14.3. GitHub upstream