SQL Trigger To Update

Question

Good day I have an external Program that stores information in SQL, I am trying to do a trigger that Updates a table when some of the fields in that Table change.

So I have one column Contractual Amount that should be updated everytime any of the values in ufAPCHANG1EAMNT, ufAPHANGE2AMNT and ufAPCHANGE3AMNT changes.

A person can change one or all of these values and can be either +ve or -ve then if they are any changes to those fields the Contractual Amount should be updated accordingly but adding the +ve amount and subtracting the negative amount.

Please can you check my code and let me know where I am going wrong.

ALTER trigger [dbo].[trgContractualAmt] 
ON [dbo].[Vendor]
AFTER UPDATE 
AS
    declare 
        @IdI integer,
        @value1 decimal,
        @Value2 decimal,
        @value3 decimal,
        @sum decimal,
        @total decimal

    SELECT 
        @IdI = i.DCLink,
        @value1 = i.ufAPCHANGE1AMT,
        @Value2 = i.ufAPCHANGE2AMNT,
        @value3 =i.ufAPCHANGE3AMNT,
        @total = i.ufAPContAmt
    FROM 
        inserted i
BEGIN
    IF TRIGGER_NESTLEVEL() > 1
        RETURN

    IF @value1 <> (select ufAPCHANGE1AMT from Vendor where DCLink = @IdI)
            UPDATE Vendor
            SET ufAPContAmt = ufAPContAmt + @value1
            where DCLink = @IdI

        if @Value2 <> (select ufAPCHANGE2AMNT from Vendor where DCLink = @IdI)
            UPDATE Vendor
            SET ufAPContAmt = ufAPContAmt + @value2
            where DCLink = @IdI

        if @Value3 <> (select ufAPCHANGE3AMNT from Vendor where DCLink = @IdI)
            UPDATE Vendor
            SET ufAPContAmt = ufAPContAmt + @value3
            where DCLink = @IdI         
END 

Answer 1

I'll suggest to use simple query when possible:

ALTER TRIGGER dbo.trgContractualAmt ON dbo.Vendor
    AFTER UPDATE
AS
    BEGIN
        UPDATE
            V
        SET
            ufAPContAmt += CASE WHEN (V.ufAPCHANGE1AMT <> I.ufAPCHANGE1AMT)
                                THEN I.ufAPCHANGE1AMT
                                ELSE 0
                           END
            + CASE WHEN (V.ufAPCHANGE2AMNT <> I.ufAPCHANGE2AMNT)
                   THEN I.ufAPCHANGE2AMNT
                   ELSE 0
              END
            + CASE WHEN (V.ufAPCHANGE3AMNT <> I.ufAPCHANGE3AMNT)
                   THEN I.ufAPCHANGE3AMNT
                   ELSE 0
              END
        FROM
            Vendor V
        INNER JOIN inserted I
            ON V.DCLink = I.DCLink;

    END;