Question

I have a rather complex query that pretty much mimics a test query I have below:

SELECT C.*
  FROM Customer C 
       INNER JOIN CustDetail CD ON C.CustomerId = CD.CustomerId
       INNER JOIN Address A ON CD.DetailID = A.DetailID
       INNER JOIN Group G ON C.CustomerId = G.CustomerId   --Join only when C.code = 1
       INNER JOIN GroupDetail D ON G.GroupId = D.DetailId  --Join only when C.code = 1
WHERE G.Active = 1 AND        --Only when C.code = 1
      D.code = '1' AND        --Only when C.code = 1
      C.Id = @customerId

I’d like to do INNER JOINs on Group G and GroupDetail D (and ofcourse not have them in the WHERE conditions based on the table column C.code = 1

I replaced the INNER JOINs with LEFT OUTER JOINs for both the join conditions, but the result set is not what was expected

How do I conditionally do the JOIN

Answer 1

SELECT C.*
  FROM Customer C 
       INNER JOIN CustDetail CD ON C.CustomerId = CD.CustomerId
       INNER JOIN Address A ON CD.DetailID = A.DetailID
       LEFT OUTER JOIN Group G ON C.CustomerId = G.CustomerId
       LEFT OUTER JOIN GroupDetail D ON G.GroupId = D.DetailId
WHERE ((G.Active = 1 AND C.code = 1) OR G.Active IS NULL) AND
      ((D.code = '1' AND C.code = 1) OR D.code IS NULL) AND
      C.Id = @customerId

I’m guessing you didn’t include the IS NULL checks before so you never got to see rows where C.code <> 1 ?

You should check for NULL on a field that will never be null. This is almost always ‘id’, but it’s not clear that you have a G.id or a D.id.