COUNT and MAX/MIN and (HAVING?) in one query/ AdventureWorks2017 task

I have a training task in AdventureWorks2017 DB. The task is the following: I need a list of the Jobtitle in which the fewest and most women work in proportions. Only consider those in which at least 4 people work.

So far my code looks like this:

select a.JobTitle, AVG(ertek) as avg_women, COUNT(*) as sum_women
from
(select BusinessEntityID, JobTitle, Gender, CASE WHEN Gender = 'F' THEN 1.0 ELSE 0.0
END as ertek
from HumanResources.Employee) a
group by a.JobTitle

This gives me back all the JobTitles, the avarage of women in those titles and the sum of the women in the jobtitles. The problem is that I cant use MAX and MIN in the query (Maybe I need to create another subquery?). And I need to get the last part of the task "Only consider those in which at least 4 people work." Thanks for the help!

ER diagram: https://homel.vsb.cz/~dan11/ddj/AdventureWorks2008_db_diagram.pdf

Take these kind of requirements one at a time and build your solution gradually.

Run your query after each additional step to confirm the expected result!

Steps

1. Only job titles with more than 4 people:
   group by e.JobTitle having count(1) >= 4
2. Count women proportionally (I interpret this as "the relative number of women within one job title"). This requires the overall count and the women only count:
   count(1) as JobTitleCount and
   count(case when e.Gender = 'F' then 1 end) as FemaleCount
   The relative (proportional) number or percentage then becomes:
   count(case when e.Gender = 'F' then 1 end)*100.0/count(1) as FemalePercentage
3. "Fewest" and "most" means ranking functions with an order by clause:
   dense_rank() over(order by <percentage from step 2 comes here> ) as RankLeast and
   dense_rank() over(order by <percentage from step 2 comes here> desc) as RankMost
4. Filter on the job titles that are in first place for either ranking:
   where jfpr.RankLeast = 1 or jfpr.RankMost = 1

Intermediate result

Steps 1. to 3.

select  e.JobTitle,
        count(1) as JobTitleCount,
        count(case when e.Gender = 'F' then 1 end) as FemaleCount,
        count(case when e.Gender = 'F' then 1 end)*100.0/count(1) as FemalePercentage,
        dense_rank() over(order by count(case when e.Gender = 'F' then 1 end)*100.0/count(1)     ) as FemalePercentageRankLeast,
        dense_rank() over(order by count(case when e.Gender = 'F' then 1 end)*100.0/count(1) desc) as FemalePercentageRankMost
from HumanResources.Employee e
group by e.JobTitle
having count(1) >= 4
order by FemalePercentage;

Full solution

Moving the entire previous query in a subquery jfpr and leaving out the columns we no longer need for the final result (JobTitleCount and FemaleCount). The order by moves to outside the subquery.

select  jfpr.JobTitle,
        jfpr.FemalePercentage
from (  select  e.JobTitle,
                count(case when e.Gender = 'F' then 1 end)*100.0/count(1) as FemalePercentage,
                dense_rank() over(order by count(case when e.Gender = 'F' then 1 end)*100.0/count(1)     ) as FemalePercentageRankLeast,
                dense_rank() over(order by count(case when e.Gender = 'F' then 1 end)*100.0/count(1) desc) as FemalePercentageRankMost
        from HumanResources.Employee e
        group by e.JobTitle
        having count(1) >= 4 ) jfpr -- job female percentage rank
where jfpr.FemalePercentageRankLeast = 1
   or jfpr.FemalePercentageRankMost = 1
order by jfpr.FemalePercentage;

Result

On my copy of AdventureWorks (no idea what version I have right now) this produces:

JobTitle                      FemalePercentage
----------------------------  ----------------
Quality Assurance Technician   0.000000000000
Scheduling Assistant           0.000000000000
Janitor                       50.000000000000
Application Specialist        50.000000000000