SQL group function nested too deeply - sql

Question

I'm trying to create an sql query that will return the smallest occurrence of an id appearing between two tables however I keep getting the error with the line HAVING MIN(COUNT(E.C_SE_ID)). Oracle is saying that the group by function is nested too deeply.
I cannot think of another way of returning C_SE_ID
SELECT CS.C_SE_ID, MIN(COUNT(E.C_SE_ID))
FROM COURSE_SECTION CS, ENROLLMENT E, LOCATION L
WHERE CS.C_SE_ID=E.C_SE_ID
AND CS.LOC_ID=L.LOC_ID
AND L.BLDG_CODE='DBW'
GROUP BY CS.C_SE_ID
HAVING MIN(COUNT(E.C_SE_ID));
in enrollment table s_id and c_se_id are linked, I'm trying to get all the s_id that are related to that c_se_id. with the updated query oracle doesn't like the select * (for obvious reasons) but when I change it too e.c_Se_id I get nothing.
SELECT E.S_ID
FROM COURSE_SECTION CS, ENROLLMENT E
WHERE CS.C_SE_ID=E.C_SE_ID
AND E.C_SE_ID =(
select *
from (select CS.C_SE_ID, count(*) as cnt,
max(count(*)) over (partition by cs.c_se_id) as maxcnt
from COURSE_SECTION CS join
ENROLLMENT E
on CS.C_SE_ID=E.C_SE_ID join
LOCATION L
on CS.LOC_ID=L.LOC_ID
where L.BLDG_CODE='DBW'
GROUP BY CS.C_SE_ID
order by count(*) desc
) t
where cnt = maxcnt);

Answer 1

One way to do this is by nesting your query and then choosing the first row in the output:
select C_SE_ID, cnt
from (select CS.C_SE_ID, count(*) as cnt
from COURSE_SECTION CS join
ENROLLMENT E
on CS.C_SE_ID=E.C_SE_ID join
LOCATION L
on CS.LOC_ID=L.LOC_ID
where L.BLDG_CODE='DBW'
GROUP BY CS.C_SE_ID
order by count(*) desc
) t
where rownum = 1
Note I updated the join syntax to the more modern version using on instead of where.
If you want all minimum values (and there are more than one), then I would use analytic functions. It is a very similar idea to your original query:
select *
from (select CS.C_SE_ID, count(*) as cnt,
max(count(*)) over (partition by cs.c_se_id) as maxcnt
from COURSE_SECTION CS join
ENROLLMENT E
on CS.C_SE_ID=E.C_SE_ID join
LOCATION L
on CS.LOC_ID=L.LOC_ID
where L.BLDG_CODE='DBW'
GROUP BY CS.C_SE_ID
order by count(*) desc
) t
where cnt = maxcnt;
Try this instead of your original query:
SELECT E.S_ID
FROM ENROLLMENT E
where E.C_SE_ID in (select C_SE_ID
from (select CS.C_SE_ID, count(*) as cnt,
max(count(*)) over (partition by cs.c_se_id) as maxcnt
from ENROLLMENT E
LOCATION L
on CS.LOC_ID=L.LOC_ID
where L.BLDG_CODE='DBW'
GROUP BY e.C_SE_ID
) t
where cnt = maxcnt)
);
In addition to fixing the joins, I also removed all references to course_section. This table doesn't seem to be used (unless for filtering results), and removing it implifies the queries.