subquery exercise - sql

Question

I need to write a query that contains a subquery where it would list the name of departments and the number of employees per department having the word 'Representative' in their job_title and the list must be ordered by department_id.
I've written this query
SELECT d.department_name, emp.employee_id
FROM departments d, employees emp, jobs j
WHERE emp.department_id=d.department_id
AND j.job_title LIKE '%Representative%';

Answer 1

If there is a requirement to use a subquery, then the following will achieve what you're looking for:
select d.department_id,
d.department_name,
(select count(*)
from employees emp
join jobs j
on j.job_id = emp.employee_job -- I've made some assumptions, here!
where emp.department_id = d.department_id
and j.job_title like '%Representative%') reps
from departments d
order by d.department_id;
Personally, however, I would use a query like this:
select d.department_id,
d.department_name,
count(emp.employee_id) reps
from departments d
join employees emp
on emp.department_id = d.department_id
join jobs j
on j.job_id = emp.employee_job -- Same assumption as before!
where j.job_title like '%Representative%'
group by d.department_id,
d.department_name
order by d.department_id;
I find it easier to read/interpret, but that's ultimately up to you.

Answer 2

You don't need a subquery for this. Use a simple JOIN.
SELECT d.department_name, COUNT(*) AS cnt
FROM employee e JOIN department d
ON e.department_id = d.department_id
JOIN jobs j ON e.jobid = j.jobid
WHERE j.job_title LIKE '%Representative%'
GROUP BY d.department_name
Or, if it is just an exercise for you, I would suggest using the following query:
SELECT d.department_name
, (SELECT COUNT(e.*)
FROM employees e JOIN jobs j ON e.jobid = j.jobid
WHERE e.department_id = d.department_id
AND j.job_title LIKE '%Representative%') AS cnt
FROM departments d
In the real world, however, you code for convenience, not just for exercise. Your code should be convenient to read, understand and maintain for all those who are involved in your software development process. If it is just an exercise then you can use the second query. But if you have to use the query in a live application, the approach in the first query is better for everyone around you.

Answer 3

There is some join logic missing in your example, but something like this may work for you:
select d.department_name, count(emp.employee_id)
from departments d, employees emp, jobs j
where j.job_title in (select job_title from jobs where job_title like '%Representative%')
group by d.department_name
The SQL may not be 100% correct but you can see the point. it's hard to complete it without all of the join logic.
This should return you all of the department names and the employee count where the employee job title contains representative.