Find the manager_id with most people under supervision - sql

Question

I am having trouble displaying the most people under manager_id. The answer is manager_id = 100 but I can't seem to make a sql that displays it. Below are 2 tables that were created and given to me.
CREATE TABLE departments
( department_id NUMBER(4)
, department_name VARCHAR2(30)
CONSTRAINT dept_name_nn NOT NULL
, manager_id NUMBER(6)
, location_id NUMBER(4)
) ;
CREATE TABLE employees
( employee_id NUMBER(6)
, first_name VARCHAR2(20)
, last_name VARCHAR2(25)
CONSTRAINT emp_last_name_nn NOT NULL
, email VARCHAR2(25)
CONSTRAINT emp_email_nn NOT NULL
, phone_number VARCHAR2(20)
, hire_date DATE
CONSTRAINT emp_hire_date_nn NOT NULL
, job_id VARCHAR2(10)
CONSTRAINT emp_job_nn NOT NULL
, salary NUMBER(8,2)
, commission_pct NUMBER(2,2)
, manager_id NUMBER(6)
, department_id NUMBER(4)
, CONSTRAINT emp_salary_min
CHECK (salary > 0)
, CONSTRAINT emp_email_uk
UNIQUE (email)
) ;
Below is my code where I am trying to join the two tables employees and departments together to find manager_id between them with the most occurrence.
Every time I try to run my sql block it gives me an error like "ORA-00918: column ambiguously defined" or something is wrong with Limit 1
SELECT COUNT(Manager_id) into v_manager_id,
FROM departments d
RIGHT JOIN employees e
ON d.manager_id = e.manager_id
GROUP BY Manager_id
ORDER BY COUNT(Manager_id) DESC
LIMIT 1;

Answer 1

You should rather select count(employee_id) ... group by manager_id so for all employees who are under some manager that count is displayed and then check if that count is max under that manager or not
else it is alias or qualifier issue you should name manager_id as some alias.

Answer 2

this will work:
select manager_id
from (select manager_id,count(*)
from employees
group by manager_id
order by
count(*) desc)
where rownum<=1 ;
you can also use nested subquery like this:
create table ns_231(col1 number,col2 number);
insert into ns_231 values(1,1);
insert into ns_231 values(2,3);
insert into ns_231 values(3,3);
insert into ns_231 values(1,2);
insert into ns_231 values(2,5);
insert into ns_231 values(2,1);
insert into ns_231 values(3,1);
insert into ns_231 values(1,4);
SELECT * FROM ns_231;
commit;
select col1 from (select col1,count(*) from ns_231 group by col1 order by count(*) desc) where rownum<=1 ;
select col1 from ns_231 group by col1
having count(*)=(select max(total) from (select count(*) as total from
ns_231 group by col1));
output:
1
2
for your table the query is :
select manager_id from employees group by manager_id
having count(*)=(select max(total) from (select count(*) as total from
employees group by manager_id));

Answer 3

I think you need to add qualifier to manager_id because it occurs in both tables
SELECT COUNT(d.Manager_id) into v_manager_id,
FROM departments d
RIGHT JOIN employees e
ON d.manager_id = e.manager_id
GROUP BY d.Manager_id
ORDER BY COUNT(d.Manager_id) DESC
LIMIT 1;