How to get two different total in two different tables in firebird? (Joining 3 Tables using left outer join) - sql

Question

I have 3 tables and the sample data is listed below
Table #1 EMPLOYEE
EMPLOYEE ID EMPLOYEE_NAME
1 Juan Dela Cruz
2 Jobert Saver
Table #2 ADD_TABLE
ADD_TABLE_PK EMPLOYEE_ID ADD_AMOUNT
1 1 10.00
2 1 13.00
Table #3 SUBTRACT_TABLE
SUBTRACT_PK EMPLOYEE_ID SUBTRACT_AMOUNT
1 1 2.00
2 1 3.00
3 1 4.00
My desired output is just like this:
SELECT PROCEDURE
EMPLOYEE_ID EMPLOYEE_NAME TOTAL(Total = sum of ADD_TABLE.ADD_AMOUNT - sum of SUBTRACT_TABLE.SUBTACT_AMOUNT)
1 Juan Dela Cruz 14.00 ( = 23.00 - 9.00)
but the result of my select procedure is:
EMPLOYEE_ID EMPLOYEE_NAME TOTAL
1 Juan Dela Cruz 51.00
here is my select procuder using left outer join
CREATE PROCEDURE SAMPLE_SELECT
RETURNS(
EMPLOYEE_ID INTEGER,
EMPLOYEE_NAME VARCHAR(50) CHARACTER SET ISO8859_1 COLLATE ISO8859_1,
TOTAL DECIMAL(12, 2))
AS
BEGIN
FOR
SELECT
A.EMPLOYEE_ID,
A.EMPLOYEE_NAME,
SUM(B.ADD_AMOUNT) - SUM(C.SUBTRACT_AMOUNT)
FROM EMPLOYEE A
LEFT OUTER JOIN ADD_TABLE B ON A.EMPLOYEE_ID = B.EMPLOYEE_ID
LEFT OUTER JOIN SUBTRACT_TABLE C ON A.EMPLOYEE_ID = C.EMPLOYEE_ID
GROUP BY
A.EMPLOYEE_ID,
A.EMPLOYEE_NAME
INTO
:EMPLOYEE_ID,
:EMPLOYEE_NAME,
:TOTAL
DO
BEGIN
SUSPEND;
END
END;

Answer 1

You need to do the aggregation before joining the tables:
select e.*, coalesce(add_amount, 0) - coalesce(subtract_amount, 0)
from employee e left join
(select employee_id, sum(add_amount) as add_amount
from add_table a
group by employee_id
) a
on a.employee_id = e.employee_id left join
(select employee_id, sum(subtract_amount) as subtract_amount
from subtract_table s
group by employee_id
) s
on s.employee_id = e.employee_id;
Note: This will keep all employees. Your query attempt uses LEFT JOINs, so this does as well.

Answer 2

Tested with Firebird 2.5, more direct:
Select e1.employee_id, e1.employee_name,
(Select Coalesce(Sum(a1.add_amount), 0)
From add_table a1
Where a1.employee_id = e1.employee_id) -
(Select Coalesce(Sum(s1.subtract_amount), 0)
From subtract_table s1
Where s1.employee_id = e1.employee_id) As total
From employee e1