SQL SELECT Sum værdier uden at inkludere dubletter

Du bør bruge underforespørgsler på int og ext til at summere og slutte sig til underforespørgslerne.

Jeg foreslår også at bruge eksplicitte JOINs i stedet for tabel, tabel, tabel

f.eks.

SELECT emp.Name, int.Amount AS InternalCostTotal, ext.Amount AS ExternalCostTotal
FROM Employees emp
JOIN ( 
    SELECT EmployeeID, SUM(Amount) AS Amount 
    FROM InternalCosts 
    GROUP BY EmployeeID 
) int ON emp.EmployeeId = int.EmployeeID
JOIN ( 
    SELECT EmployeeID, SUM(Amount) AS Amount 
    FROM ExternalCosts 
    GROUP BY EmployeeID 
) ext ON emp.EmployeeId = ext.EmployeeID