Select rows based on count of child table - sql-server

Question

I have three entities: department, employee, and report. A department has many employees, each of whom has many reports. I want to select the one employee in each department who has the most reports. I have no idea how to even start this query. This question seems very similar, but I can't figure out how to manipulate those answers for what I want.
I have full access to the entire system, so I can make any changes necessary. In the event of a tie, it's safe to arbitrarily pick one of the results.
Department:
ID | Name
----|------
1 | DeptA
2 | DeptB
3 | DeptC
4 | DeptD
Employee:
ID | Name | DeptID
----|------|--------
1 | Joe | 1
2 | John | 1
3 | Emma | 2
4 | Jack | 3
5 | Sven | 3
6 | Axel | 4
7 | Brad | 4
8 | Jane | 4
Report:
ID | EmployeeID
----|------------
1 | 1
2 | 2
3 | 3
4 | 5
5 | 6
6 | 6
7 | 8
Desired result (assuming I queried names only):
Joe OR John (either is acceptable)
Emma
Sven
Axel

Answer 1

How to start this query? Well, get the information about each employee, the department, and the number of reports:
select e.name, e.deptid, count(*) as numreports
from employee e join
reports r
on e.id = r.employeeid
group by e.name, e.deptid;
Now you just want the largest count in each department. I would suggest row_number() or rank() depending on how you want to handle ties:
select er.*
from (select e.name, e.deptid, count(*) as numreports,
row_number() over (partition by e.deptid order by count(*) desc) as seqnum
from employee e join
reports r
on e.id = r.employeeid
group by e.name, e.deptid
) er
where seqnum = 1;
If you want the department name instead of number, you can join that in as well.

Answer 2

From your Question schema will be
SELECT * into #Department FROM(
select 1 ID,'DEPTA' NAME
UNION ALL
select 2,'DEPTB'
UNION ALL
select 3,'DEPTC'
UNION ALL
select 4,'DEPTD')TAB
SELECT * INTO #Employee FROM (
SELECT 1 ID ,'Joe' Name , 1 DeptID
UNION ALL
SELECT 2 , 'John' , 1
UNION ALL
SELECT 3 , 'Emma' ,2
UNION ALL
SELECT 4 ,'Jack' , 3
UNION ALL
SELECT 5 ,'Sven' , 3
UNION ALL
SELECT 6 , 'Axel' , 4
UNION ALL
SELECT 7 ,'Brad' , 4
UNION ALL
SELECT 8 ,'Jane' , 4)AS A
SELECT * INTO #Report FROM(
SELECT 1 ID ,1 EmployeeID
UNION ALL
SELECT 2, 2
UNION ALL
SELECT 3 ,3
UNION ALL
SELECT 4, 5
UNION ALL
SELECT 5, 6
UNION ALL
SELECT 6, 6
UNION ALL
SELECT 7, 8
UNION ALL
SELECT 8, 8
UNION ALL
SELECT 9, 8
)AS A
And you need to apply DENSE_RANK() for giving rank based on no of reports(count)
;WITH CTE AS(
select DEP.ID DEP_ID, DEP.NAME DEP,EMP.ID EMP_ID, EMP.Name EMP
,DENSE_RANK() OVER(PARTITION BY DEP.ID ORDER BY COUNT(REP.ID) DESC) REP_RANK
,COUNT(REP.ID) NO_OF_REP FROM #Department DEP
inner join #Employee emp on emp.deptid=dep.id
inner join #report rep on rep.EmployeeID=emp.id
GROUP BY DEP.ID, DEP.NAME ,EMP.ID, EMP.Name
)
SELECT DEP, EMP, NO_OF_REP FROM CTE WHERE REP_RANK=1
Here in the DEPTA Joe & John both will be picked because both are having 1 report count which is a max count in DEPTA.
And the result will be
+-------+------+-----------+
| DEP | EMP | NO_OF_REP |
+-------+------+-----------+
| DEPTA | Joe | 1 |
| DEPTA | John | 1 |
| DEPTB | Emma | 1 |
| DEPTC | Sven | 1 |
| DEPTD | Jane | 3 |
+-------+------+-----------+

Answer 3

Please try the below code:-
SELECT D.NAME
FROM (
SELECT C.NAME, RANK() OVER (
PARTITION BY C.DEPTID ORDER BY C.COUNTS DESC
) RNK
FROM (
SELECT EMPID, NAME, COUNT(EMPID) AS COUNTS, DEPTID
FROM DBO.REPORT AS A
JOIN DBO.EMPLO AS B ON A.EMPID = B.ID
GROUP BY EMPID, NAME, DEPTID
) AS C
) AS D
WHERE D.RNK = 1