Group By with MAX value from another column - sql

Question

Table FieldStudies is :
ID Name
---|-----------------------|
1 | Industrial Engineering|
2 | Civil Engineering |
3 | Architecture |
4 | Chemistry |
And table Eductionals is :
ID UserID Degree FieldStudy_ID
---|------|--------|------------|
1 | 100 | 3 | 4 |
2 | 101 | 2 | 2 |
3 | 101 | 3 | 2 |
4 | 101 | 4 | 3 |
5 | 103 | 3 | 4 |
6 | 103 | 4 | 2 |
I want to find the number of students in each FieldStudies , provided that the highest Degree is considered.
Output desired:
ID Name Count
---|-----------------------|--------|
1 | Industrial Engineering| 0 |
2 | Civil Engineering | 0 |
3 | Architecture | 1 |
4 | Chemistry | 2 |
I have tried:
select Temptable2.* , count(*) As CountField from
(select fs.*
from FieldStudies fs
left outer join
(select e.UserID , Max(e.Degree) As ID_Degree , e.FieldStudy_ID
from Eductionals e
group by e.UserID) Temptable
ON fs.ID = Temptable.FieldStudy_ID) Temptable2
group by Temptable2.ID
But I get the following error :
Column 'Eductionals.FieldStudy_ID' is invalid in the select list
because it is not contained in either an aggregate function or the
GROUP BY clause.

Answer 1

If I understand correctly, you want only the highest degree for each person. If so, you can use row_number() to whittle down the multiple rows for a given person and the rest is aggregation and join:
select fs.id, fs.Name, count(e.id)
from fieldstudies fs left join
(select e.*,
row_number() over (partition by userid order by degree desc) as seqnum
from educationals e
) e
on e.FieldStudy_ID = fs.id and seqnum = 1
group by fs.id, fs.Name
order by fs.id;