Getting a derived attribute through two different tables - database

Question

I need to get the salary of an employee through two different tables: his gains and his discounts. The relation from the table employee to the two tables is a many to many relation. So I need to take the employee_id, get all the gain_id in the employee_gains table, add all of them and subtract with the analogue result in the discounts.
I tried this creating a view for the salary:
CREATE VIEW salary as
select ((select sum(value) from gains
where gain_id in (select gain_id from gain_employee where employee_id=2))
-
(select sum(value) from discount
where discount_id in (select gain_id from discount_employee where employee_id=2)));
However, this only (and successfully) gives me the salary for the employee with ID 2. But how can I make this generic? I want a view salary for all the employees.

Answer 1

I would suggest you to use two CTEs to calculate the gains and discounts and then do a FULL OUTER JOIN on the two sets. This will ensure that you get proper values such as 0 for missing gains or discounts for an employee_id. If you want to ignore them such cases just change it to a plain INNER JOIN
CREATE OR REPLACE VIEW V_salary AS --give proper name to indicate it's a view
WITH ge
AS (SELECT e.employee_id,
SUM(g.value) AS gain_value
FROM gain_employee e
JOIN gains g --use left join if some employees don't
--have an entry in gains
ON e.gain_id = g.gain_id
GROUP BY e.employee_id),
de
AS (SELECT e.employee_id,
SUM(d.value) AS dis_value
FROM discount_employee e
JOIN discounts d --use left join if some employees don't
--have an entry in discount
ON e.discount_id = d.discount_id
GROUP BY e.employee_id)
SELECT COALESCE(ge.employee_id, gd.employee_id), --gets you atleast one of
--them when one may be missing.
COALESCE(ge.gain_value, 0) - COALESCE(de.dis_value, 0) AS salary
FROM ge
FULL OUTER JOIN de -- to consider case where one of them is absent
ON ge.employee_id = de.employee_id;

Answer 2

This should do it -
CREATE VIEW salary as
select S1.employee_id, (S1.gains - S2.discounts) as salary
from (select ge.employee_id, sum(g.value) as gains
from gain_employee ge, gains g
where ge.gain_id = g.gain_id
group by ge.employee_id) S1,
(select de.employee_id, sum(d.value) as discounts
from discount_employee de, discounts d
where de.doscount_id = d.discount_id
group by de.employee_id) S2
where S1.employee_id = S2.employee_id;
Then you can query this view with employee_id as the condition.