LIMIT by GROUP in SQl (postgresql) - sql

Question

Below is my table students (having 100K+ rows in orignal, showing just a set):
id
name
class
marks
1
Ryan
5
8
2
Max
5
7
3
Max1
5
10
4
Ryan1
6
8
5
Max2
6
10
6
Ryan2
6
7
7
Ryan3
7
8
8
Max3
7
10
9
Ryan4
7
7
I want to fetch two rows per class ( 5 & 7) having marks <= 10 , also sorted by class, marks ASC
So, the expected result will be:-
id
name
class
marks
1
Ryan
5
8
3
Max1
5
10
7
Ryan3
7
8
8
Max3
7
10
To execute below I tried:-
SELECT DISTINCT t_out.class, t_top.marks, t_top.name
FROM students t_out
JOIN LATERAL (
select *
from students t_in
where t_in.class = t_out.class
ORDER BY t_in.id ASC
) t_top ON TRUE
WHERE t_top.marks <= 10
AND (t_out.class = 5 OR t_out.class = 7)
ORDER BY t_top.marks DESC LIMIT 2;
Result on original database:- it's loading since long time
Result on sample :- Error: near line 20: near "(": syntax error
Is 10 the highest marks?

Answer 1

You would use row_number():
select s.*
from (select s.*,
row_number() over (partition by class order by marks desc) as seqnum
from students s
where marks < 10 and class in (5, 7)
) s
where seqnum <= 2
order by class, marks;
Note: Your question is a little confusing. You seem to want two rows with the highest marks per class ordered in descending order by marks.
EDIT:
Based on your comment:
select s.*
from (select s.*,
row_number() over (partition by class order by marks desc) as seqnum,
count(*) over (partition by class) as cnt
from students s
where marks < 10
) s
where seqnum <= 2 and cnt >= 2
order by class, marks;