How to get sum of the same Unit's - sql

Question

I have a table which contains a college's departments and their units and sub-units.
OrganizationID ParentUnit Unit ChildUnit UnitName
10 1 0 0 Education
12 1 1 0 Sports
24 1 2 0 Mathmatics
28 1 3 0 Science
35 1 3 1 Physics
51 1 4 0 Arts
66 1 4 1 Music
69 1 4 2 Painting
84 8 0 0 Business & Administration
88 8 1 0 Administration
96 8 1 1 Public Administration
107 8 1 2 Local Managements
110 8 2 0 Finance
119 8 2 1 Accounting
124 8 2 2 Marketing
I have another table which contains the student information of that college.
StudentID OrganizationID
1 12
2 12
3 24
5 28
6 35
8 51
9 66
31 69
34 96
45 88
57 96
66 107
69 110
72 69
74 124
I want to get student counts for each unit. If a studutent's Organization is a ChildUnit it should be added to current Unit. If ChildUnit is greater than0 corresponding student count should be added to same Unit For example Physics is a child of Science. Then Science student count should return 2.
My target data table should look like as the following
ParentUnit UnitName StudentCount
------------------------------------------------------
Education Sports 2
Education Mathmatics 1
Education Science 2
Education Arts 4
Business & Administration Administration 4
Business & Administration Finance 2
I have done it in programmatic way. There are many for and if loops. Then I started to think whether it could be done with a smarter sql query.

Answer 1

That doesn't look so difficult. You are looking for the student count per ParentUnit + Unit. Then the name for such a group is the record where the level (the ChildUnit) is zero. You get that record with a CASE construct, then use MIN or MAX, because you need an aggregate function here (there should be exactly one record per group anyhow, so MIN = MAX).
select
min(case when o.childunit = 0 then o.unitname end) as unitname,
count(*) as studentcount
from organization o
inner join student s on s.organizationid = o.organizationid
group by o.parentunit, o.unit;
To include the parent unit name:
select
(
select unitname
from organization po
where po.parentunit = o.parentunit
and po.unit =0
and po.childunit = 0
) as parentunitname,
min(case when o.childunit = 0 then o.unitname end) as unitname,
count(*) as studentcount
from organization o
inner join student s on s.organizationid = o.organizationid
group by o.parentunit, o.unit;
Or:
select
min(po.unitname) as parentunitname,
min(case when o.childunit = 0 then o.unitname end) as unitname,
count(*) as studentcount
from organization o
inner join student s on s.organizationid = o.organizationid
inner join
(
select parentunit, unitname
from organization
where unit = 0 and childunit = 0
) po on po.parentunit = o.parentunit
group by o.parentunit, o.unit;