如何在firebird的两个不同表中获得两个不同的总数？ （使用左外部联接联接3个表）

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 JOIN s, 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