SQL - Join issue

Question

I have the following query:

SELECT rt.ID, rt.Name, rt.Rate, rt.Colour, vtb.ID AS 'vtbID', vtb.Value, rt.StdID
FROM Rates AS rt 
LEFT OUTER JOIN VehicleTypeCostsBreakdown AS vtb ON rt.ID = vtb.RateID
LEFT OUTER JOIN VehicleTypeCostsDepots AS vtd ON vtd.ID = vtb.VehicleTypeDepotID AND vtd.DepotID = @DepotID AND vtd.VehicleTypeID = @VehicleTypeID

Basically, I want to select all 'rates' from Rates table, but if any references to a rate exists in the 'vtd' table, which has parameters that match @DepotID and @VehicleTypeID, I want to bring back the Value for that. If it doesn't have any referenced, I want it the 'vtb.Value' selection to be blank.

With the SQL above, it seems to always return a value for 'vtb.Value' value, even if the parameters are null. Am I missing something?

Answer 1

Try it this way. Basically, you'll LEFT JOIN to the derived table formed by the INNER JOIN between VehicleTypeCostsBreakdown and VehicleTypeCostsDepots. The INNER JOIN will only match when all of your conditions are true.

SELECT rt.ID, rt.Name, rt.Rate, rt.Colour, vtb.ID AS 'vtbID', vtb.Value, rt.StdID
    FROM Rates AS rt 
        LEFT OUTER JOIN VehicleTypeCostsBreakdown AS vtb 
            INNER JOIN VehicleTypeCostsDepots AS vtd 
                ON vtd.ID = vtb.VehicleTypeDepotID 
                    AND vtd.DepotID = @DepotID 
                    AND vtd.VehicleTypeID = @VehicleTypeID
            ON rt.ID = vtb.RateID