Question

Simple academic project. Simple procedure. If there is something already in Payments table – then take debt value from that table, if there are no values – take it from Tariff table. But why such conidtion doesn’t work?

ALTER PROCEDURE dbo.GetDebt
    (
    @LoanId int
    )
AS

    IF NOT EXISTS (SELECT top 1 * FROM Payment WHERE LoanId = @LoanId) 
    BEGIN
        SELECT (TotalLoan + ( ( TotalLoan / 100 ) * Interest))  as Debt FROM Loan L, Tariff T
        WHERE L.TariffIf = L.TariffId
    END
    ELSE
    BEGIN
        SELECT MIN(Debt) as Debt FROM Loan L 
        RIGHT OUTER JOIN Payment P -- Joins are cool.
        ON L.LoanId = P.LoanId
        WHERE P.LoanId = @LoanId 
    END

Answer 1

If/Else is almost always the entirely wrong approach for sql code. It’s hard to give you an exact example without knowing more about your tables, but you really want something more like this:

SELECT COALESCE(P.Debt, TotalLoan + ( ( TotalLoan / 100 ) * Interest))  as Debt
FROM Loan L
LEFT JOIN Tariff T ON T.LoanID = L.LoanID
LEFT JOIN (SELECT LoanID, Min(Debt) As Debt FROM Payment GROUP BY LoanID) P
WHERE L.LoanID = @LoanID

No If/Else required.