Getting department names that have maximum staff count - sql

Question

I have 2 tables, one table storing details of staff (columns are staff_id, staff_name, department_id) and another table storing details of department (columns are department_id, department_name, department_block_num).
I need to write a query to display names of department that has maximum staff count order by department_name. It is also given that multiple department can also have staff count same as maximum staff count. Another condition is group function is not allowed.
Here is code
SELECT department_name
FROM department
WHERE department_id IN (SELECT department_id
FROM ( SELECT department_id, COUNT (*) AS cnt1
FROM staff
WHERE COUNT (*) =
(SELECT cnt
FROM ( SELECT department_id,
COUNT (*) AS cnt
FROM staff
GROUP BY department_id
ORDER BY cnt DESC)
WHERE ROWNUM = 1)
GROUP BY department_id));

Answer 1

You can use the RANK analytic function to find the rows with the maximum counts:
SELECT department_id,
COUNT(*) AS cnt,
RANK() OVER ( ORDER BY COUNT(*) DESC ) AS rnk
FROM staff
GROUP BY department_id
Will rank the rows in order of DESCending count of members of staff and then you can just filter on rows where the rank is 1 (and have the highest count).
Oracle Setup:
CREATE SEQUENCE staff__id__seq;
CREATE TABLE departments (
id INT PRIMARY KEY,
department_name VARCHAR2(20)
);
INSERT INTO departments ( id, department_name )
SELECT 1, 'Aaa' FROM DUAL UNION ALL
SELECT 2, 'Bbb' FROM DUAL UNION ALL
SELECT 3, 'Ccc' FROM DUAL UNION ALL
SELECT 4, 'Ddd' FROM DUAL;
CREATE TABLE staff (
id INT PRIMARY KEY,
department_id INT REFERENCES departments( id )
);
INSERT INTO staff ( id, department_id )
SELECT staff__id__seq.NEXTVAL, department_id
FROM (
SELECT 1 AS department_id FROM DUAL CONNECT BY LEVEL <= 3 UNION ALL
SELECT 2 FROM DUAL CONNECT BY LEVEL <= 5 UNION ALL
SELECT 3 FROM DUAL CONNECT BY LEVEL <= 2 UNION ALL
SELECT 4 FROM DUAL CONNECT BY LEVEL <= 5
);
Query to count staff:
SELECT department_id,
COUNT(*) AS cnt,
RANK() OVER ( ORDER BY COUNT(*) DESC ) AS rnk
FROM staff
GROUP BY department_id
DEPARTMENT_ID | CNT | RNK
------------: | --: | --:
2 | 5 | 1
4 | 5 | 1
1 | 3 | 3
3 | 2 | 4
Query 2 - Get the department name for the highest counts:
Just join the previous query to the departments table and filter to return the rows when the rank is 1 (the highest count).
SELECT d.department_name
FROM (
SELECT department_id,
RANK() OVER ( ORDER BY COUNT(*) DESC ) AS rnk
FROM staff
GROUP BY department_id
) s
INNER JOIN departments d
ON ( d.id = s.department_id )
WHERE s.rnk = 1
Output:
| DEPARTMENT_NAME |
| :-------------- |
| Bbb |
| Ddd |
db<>fiddle here

Answer 2

If you are trying to avoid group by, then use subqueries:
select d.*,
(select count(*)
from staff s
where s.department_id = d.department_id
) as staff_cnt
from department d;
If you then want the top departments, with ties, use subqueries and window functions:
select . . . -- whatever columns you want
from (select d.*,
rank() over (order by staff_cnt desc) as seqnum
from (select d.*,
(select count(*)
from staff s
where s.department_id = d.department_id
) as staff_cnt
from department d
) d
) d
where seqnum = 1;

Answer 3

Assuming this is homework or similar and that windows function are not allowed here is a solution using more basic sql
SELECT department_name
FROM department d
JOIN staff s ON d.department_id = s.department_id
GROUP BY department_name
HAVING COUNT(s.department_id) = (SELECT COUNT(*) as stat
FROM staff
GROUP BY department_id
ORDER BY stat DESC
FETCH FIRST ROW ONLY)
ORDER BY department_name