How to get SUM from two child tables

Question

this is my first posting here and i hope i made nothing wrong.

I do get wrong results with sum because i do not understand how to write my query in the correct way.

I have three tables, the first is the master and 2 identical child tables. The child tables are used for expense and income.

Table forecast

id  forecast_id
1   721     
2   721
3   721 

Table expense

forecast_id     amount      tax_free
721             100         0
721             200         1
721             300         0

Table income

forecast_id     amount      tax_free
721             50          1
721             100         0

SELECT
    t_forecast.forecastID,
    Sum(t_fc_einkauf.betrag) AS expense,
    Sum(t_fc_zahlung.betrag) AS income
FROM
    t_forecast
JOIN t_fc_einkauf ON t_fc_einkauf.forcastID = t_forecast.forecastID
JOIN t_fc_zahlung ON t_fc_zahlung.forcastID = t_forecast.forecastID
WHERE
    t_forecast.forecastID = 855
ORDER BY

My result is wrong

forcastID   expense     income
855         1200        450

It should be

forcastID   expense     income
855          600        150

Answer 1

You are using joins which creates cross product rows and therefore you are getting sum higher than expected try this with correlated subquery

SELECT
   DISTINCT f.forecast_id,
(SELECT SUM(e.amount) FROM expense e WHERE e.forecast_id = f.forecast_id )
AS expense,
 (SELECT SUM(i.amount) FROM income i WHERE i.forecast_id = f.forecast_id )
AS income
FROM
   forecast f
WHERE
    f.forecast_id = 721

Demo