Display dummy row as group by clause heading in Oracle - sql

Question

I have 3 tables say company, department and employee. Now I want to find out all the employees who works in company A under department D and want to display data as below.
There is parent key relation between all this tables
**TABLE COMPANY**
COMPANY_NAME COMPANY ID
C1 COMP1
C2 COMP2
C3 COMP3
**TABLE DEPARTMENT**
DEPARTMENT_NAME COMPANY_NAME
D1 C1
D2 C1
D3 C2
**TABLE EMPLOYEE**
EMPLOYEE_ID DEPARTMENT_NAME
E1 D1
E2 D1
E3 D1
E4 D2
E5 D2
Company -- > Department --- > Employee.
I also want to display entity against each column as dummy column.
ENTITY COMPANY_NAME DEPARTMENT_NAME EMPLOYEE_ID
COMPANY C1 - -
DEPARTMENT C1 D1
EMPLOYEE C1 D1 E1
EMPLOYEE C1 D1 E2
EMPLOYEE C1 D1 E3
DEPARTMENT C1 D2 -
EMPLOYEE C1 D2 E4
EMPLOYEE C1 D2 E5 ``
I Have tried this with group by, but with group by I get data as vertical tree like structure. but what i want is there should be heading like company or department and under that employees under that department should display. Is there are possible way we can do this in sql or plsql. ?
for left outer join
ENTITY COMPANY_NAME DEPARTMENT_NAME EMPLOYEE_ID
COMPANY C1 - -
DEPARTMENT C1 D1
EMPLOYEE C1 D1 E1
EMPLOYEE C1 D1 E2
EMPLOYEE C1 D1 E3
DEPARTMENT C1 D2 -
EMPLOYEE C1 D2 E4
EMPLOYEE C1 D2 E5
COMPANY C2 - -
DEPARTMENT C2 D3 -
COMPANY C3 - -

Answer 1

You can do this using aggregation -- and grouping sets. This looks like:
select (case when grouping(employee_id) = 0 then 'EMPLOYEE'
when grouping(department_name) = 0 then 'DEPARTMENT'
else 'COMPANY'
end) as entity,
company_name, department_name, e.employee_id
from company c join
department d
using (company_name) join
employee e
using (department_name)
group by grouping sets ( (company_name, department_name, e.employee_id), (company_name, department_name), (company_name) )
order by company_name, department_name nulls first, employee_id nulls first;
Here is a db<>fiddle.

Answer 2

You can use UNION ALL as follows:
SELECT COMPANY_NAME, NULL AS DEPARTMENT_NAME, NULL AS EMPLOYEE_ID
FROM COMPANY C
WHERE EXISTS (SELECT 1 FROM EMPLOYEE E JOIN DEPARTMENT D USING (DEPARTMENT_NAME)
WHERE C.COMPANY_NAME = D.COMPANY_NAME)
UNION ALL
SELECT COMPANY_NAME, DEPARTMENT_NAME, NULL AS EMPLOYEE_ID
FROM COMAPNY C JOIN DEPARTMENT D USING (COMPANY_NAME)
WHERE EXISTS (SELECT 1 FROM EMPLOYEE E
WHERE E.DEPARTMENT_NAME = D.DEPARTMENT_NAME)
UNION ALL
SELECT COMPANY_NAME, DEPARTMENT_NAME, EMPLOYEE_ID
FROM EMPLOYEE JOIN DEPARTMENT USING (DEPARTMENT_NAME)
ORDER BY COMPANY_NAME,
DEPARTMENT_NAME NULLS FIRST,
EMPLOYEE_ID NULLS FIRST;