SQL Query Pull data from another table IF NULL - sql

Question

I have a system running a SQL Server Express database and I need to pull some data from it. I have the basic SQL query created but I have found that some data is located elsewhere.
The basic premise is I have a database of Repair Orders, Vehicles And Customers. The Vehicles are usually added via a VIN decoder so they have ID's associated from a MAKE and MODEL table. However in the case of a VIN not decoding the application allows the user to manually enter this information and then it is stored in another table named "UserVehicleAttributes". In this table there is the VehicleID, AttributeName, & AttributeValue.
UserAttributeId VehicleId AttributeName AttributeValue
-----------------------------------------------------------
364 6829 Model Sedona
365 6830 Make Kia
366 6830 Model Sedona
So what I need is if the Make or Model comes up as NULL from the Vehicle table, I can display what as manually entered in.
I found that there is an existing function in the DB that looks to be able to do what I need but I don't know how to use it as part of my query.
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
ALTER FUNCTION [SM].[fnVehicleModelName]()
RETURNS TABLE
AS
RETURN
(
SELECT DISTINCT v.VehicleId,
CASE
WHEN v.SubModelId IS NULL THEN ISNULL(ua.[AttributeValue],'')
ELSE smm.[Name]
END as Model
FROM SM.Vehicle v (NOLOCK)
LEFT OUTER JOIN
(SELECT sm.SubModelId, m.[Name] + ' ' + sm.[Name] as Name
FROM DMV.SubModel (NOLOCK) sm
INNER JOIN DMV.Model m (NOLOCK)
ON sm.ModelId = m.ModelId ) as smm
ON v.SubModelId = smm.SubModelId
LEFT OUTER JOIN SM.UserVehicleAttributes ua (NOLOCK)--
ON v.VehicleId = ua.VehicleId and ua.AttributeName = 'Model'
Any help is greatly appreciated. I am not very good with SQL (obviously) but I am trying to figure this one out.

Answer 1

I'm not sure why you're making this a function with no parameters - that's kinda the same thing as a view. Consider if using a view here might simplify the situation.
You're correct that ISNULL is what you want to use here, but I think the join should be more simple. Your situation is basically "pull the column value from whichever table has a non-null value, giving preference to one table first"
In the outer join, all the columns from the outer joined tables will be null if there's not a match, and if there is a match, all the values should be filled in. Knowing that... you should be able to do something like this... (as an example to clarify how this concept works, not solving your query for you)
select v.VehicleId,
VehicleName = isnull(Model.Name, UserVehicle.Name)
from Vehicle v
left outer join Model on Model.VehicleID = Vehicle.VehicleID
left outer join UserVehicle on UserVehicle.VehicleID = Vehicle.VehicleId
So, what that does is join the possible rows from either table, and the ISNULL macro selects whichever value is non-null. Do that for the rest of the columns, and fix the join condition to whatever your conditions are, and you should be golden.

Answer 2

That function has no parameters, if you want to use it rewrite it as a view, but it only shows model, so you can use subqueries like this:
SELECT
VehicleId,
CASE
WHEN Make IS NULL
THEN ( SELECT AttributeValue FROM UserVehicleAttributes
WHERE VehicleId = Vehicles.VehicleId
AND AttributeName = 'Make' )
ELSE Make
END AS Make,
CASE
WHEN Model IS NULL
THEN ( SELECT AttributeValue FROM UserVehicleAttributes
WHERE VehicleId = Vehicles.VehicleId
AND AttributeName = 'Model' )
ELSE Model
END AS Model
FROM
Vehicles