Query SUM for two fields in two different tables

Question

I am trying to determine how to do sum fields between two tables.

In table 1, we\'ll call it gegevens for short, I would have, for example, gegevenID, vertrekdatum,

Answer 1

Joining in any form will double the values when there are two gegevenID rows in fees (or triple them if there are three, and so on).

The best workaround I can think of is to calculate the sums independently - one subquery for price and one subquery for fees - and then put the results together:

SELECT
  p.year,
  p.SumOfPrijs,
  f.SumOfFees,
  p.SumOfPrijs + f.SumOfFees AS Total
FROM (
  SELECT
    YEAR(vertrekdatum) AS year,
    SUM(prijs) AS SumOfPrijs
  FROM gegevens
  GROUP BY YEAR(vertrekdatum)
) p
LEFT JOIN (
  SELECT
    YEAR(vertrekdatum) as year,
    SUM(amount) AS SumOfFees
  FROM gegevens
  INNER JOIN fees ON gegevens.gegevenID = fees.gegevenID
  GROUP BY YEAR(vertrekdatum)
) f ON p.year = f.year

There's a SQL Fiddle here.

Answer 2

You need to use a subquery to aggregate the fees table before the join:

SELECT sum(prijs) as SumOfPrijs, sum(amount) as SumOfFees, sum(prijs)+sum(amount) AS   
Total, year(vertrekdatum) as year
FROM tbl_vluchtgegevens vg LEFT JOIN
     (select f.gegevenId, sum(amount) as Amount
      from tbl_fees f
      group by f.gegevenId
     ) f
     ON f.gegevenID = vg.gegevenID
WHERE vertrekdatum <=NOW()
GROUP by year(vertrekdatum);

The problem is that the multiple fees on on "gegeven" is causing the join to produce unexpected rows, that affect the sum.