SQL Query - Unsure How to Fix Logical Error - sql

Question

Edit: Sorry! I am using Microsoft SQL Server.
For clarification, you can have a department named "x" with a list of jobs, a department named "y" with a different list of jobs, etc.
I also need to use >= ALL instead of TOP 1 or MAX because I need it to return more than one value if necessary (if job1 has 20 employees, job2 has 20 employees and they are both the biggest values, they should both return).
In my query I'm trying to find the most common jobTitle and the number of employees that work under this jobTitle, which is under the department 'Research and Development'. The query I've written consists of joins to be able to return the necessary data.
The problem I am having is with the WHERE statement. The HAVING COUNT(JobTitle) >= ALL is finding the biggest number of employees that work under a job, however the problem is that my WHERE statement is saying the Department must be 'Research and Development', but the job with the most amount of employees comes from a different department, and thus the output produces only the column names and nothing else.
I want to redo the query so that it returns the job with the largest amount of employees that comes from the Research and Development department.
I know this is probably pretty simple, I'm a noob :3 Thanks a lot for the help!
SELECT JobTitle, COUNT(JobTitle) AS JobTitleCount, Department
FROM HumanResources.Employee AS EMP JOIN
HumanResources.EmployeeDepartmentHistory AS HIST
ON EMP.BusinessEntityID = HIST.BusinessEntityID JOIN
HumanResources.Department AS DEPT
ON HIST.DepartmentID = DEPT.DepartmentID
WHERE Department = 'Research and Development'
GROUP BY JobTitle, Department
HAVING COUNT(JobTitle) >= ALL (
SELECT COUNT(JobTitle) FROM HumanResources.Employee
GROUP BY JobTitle
)

Answer 1

If you only want one row, then a typical method is:
SELECT JobTitle, COUNT(*) AS JobTitleCount
FROM HumanResources.Employee AS EMP JOIN
HumanResources.EmployeeDepartmentHistory AS HIST
ON EMP.BusinessEntityID = HIST.BusinessEntityID JOIN
HumanResources.Department AS DEPT
ON HIST.DepartmentID = DEPT.DepartmentID
WHERE Department = 'Research and Development'
GROUP BY JobTitle
ORDER BY COUNT(*) DESC
FETCH FIRST 1 ROW ONLY;
Although FETCH FIRST 1 ROW ONLY is the ANSI standard, some databases spell it LIMIT or even SELECT TOP (1).
Note that I removed DEPARTMENT both from the SELECT and the GROUP BY. It seems redundant.
And, if I had to guess, your query is going to overstate results because of the history table. If this is the case, ask another question, with sample data and desired results.
EDIT:
In SQL Server, I would recommend using window functions. To get the one top job title:
SELECT JobTitle, JobTitleCount
FROM (SELECT JobTitle, COUNT(*) AS JobTitleCount,
ROW_NUMBER() OVER (ORDER BY COUNT(*) DESC) as seqnum
FROM HumanResources.Employee AS EMP JOIN
HumanResources.EmployeeDepartmentHistory AS HIST
ON EMP.BusinessEntityID = HIST.BusinessEntityID JOIN
HumanResources.Department AS DEPT
ON HIST.DepartmentID = DEPT.DepartmentID
WHERE Department = 'Research and Development'
GROUP BY JobTitle
) j
WHERE seqnum = 1;
To get all such titles, when there are duplicates, use RANK() or DENSE_RANK() instead of ROW_NUMBER().

Answer 2

with employee_counts as (
select
hist.DepartmentID, emp.JobTitle, count(*) as cnt,
case when dept.Department = 'Research and Development' then 1 else 0 end as is_rd,
from HumanResources.Employee as emp
inner join HumanResources.EmployeeDepartmentHistory as hist
on hist.BusinessEntityID = emp.BusinessEntityID
inner join HumanResources.Department as dept
on dept.DepartmentID = hist.DepartmentID
group by
hist.DepartmentID, emp.JobTitle
)
select * from employee_counts
where is_rd = 1 and cnt = (
select max(cnt) from employee_counts
/* where is_rd = 1 */ -- ??
);