Question

I am trying to perform a SELECT query on two tables joined with a LEFT JOIN where there may not be a record in the joined table. Something like:

--SELECT row using AreaID
SELECT *
FROM Rate
LEFT JOIN Area
ON Rate.AreaID = Area.AreaID
WHERE ProductID = @ProductID
AND Area.PostcodeOutcode = @PostcodeOutcode

This works when @PostcodeOutcode exists in the Area table, but I still need to return the record in the left table if there is not a record in the right table.

I am fudging it currently by doing this, but I know there is a better solution:

DECLARE @AreaID int
SELECT @AreaID = AreaID
FROM Area WHERE PostcodeOutcode = @PostcodeOutcode 

--SELECT row using AreaID
SELECT *
FROM Rate
WHERE ProductID = @ProductID
AND
(
    AreaID = @AreaID
    OR (@AreaID IS NULL AND AreaID IS NULL)
)

I know this is probably simple, but my SQL knowledge is limited. Please help.

Thanks

Alex

Answer 1

move the area check to the join

SELECT * FROM Rate
LEFT JOIN Area 
  ON Rate.AreaID = Area.AreaID and Area.PostcodeOutcode = @PostcodeOutcode
WHERE ProductID = @ProductID 

Update for the revised question in comments, Is this what you want?

SELECT Rate.RatePercent FROM Rate
INNER JOIN Area 
  ON Rate.AreaID = Area.AreaID and Area.PostcodeOutcode = @PostcodeOutcode
WHERE 
  ProductID = @ProductID
UNION ALL
SELECT Rate.RatePercent FROM Rate 
where
  ProductID = @ProductID 
and 
  AreaId is null 
and 
 not exists(select PostCodeOutCode From Area where PostCodeOutCode=@PostCodeOutcode)