Conditional INNER JOIN in SQL Server - sql

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.

Answer 2

I'm guessing what you want is just a tighter ON clause, and a compound condition.
SELECT C.*
FROM Customer C
INNER JOIN CustDetail CD ON C.CustomerId = CD.CustomerId
INNER JOIN Address A ON CD.DetailID = A.DetailID
-- the next two joins happen only when c.code=1
-- their columns will be null when there is no match.
LEFT JOIN Group G ON C.CustomerId = G.CustomerId AND C.Code = 1
LEFT JOIN GroupDetail D ON G.GroupId = D.DetailId AND C.Code = 1
WHERE C.Id = #customerId AND --always check this
-- this condition is true if code is null or code isn't 1,
((C.code IS NULL or C.code <> 1)
-- or (if the code is 1), it is true if g.active and d.code
OR (G.Active = 1 AND D.code = '1'))

Answer 3

This will do a semi-join only when code is 1.
SELECT C.*
FROM Customer C
INNER JOIN CustDetail CD
ON C.CustomerId = CD.CustomerId
INNER JOIN Address A
ON CD.DetailID = A.DetailID
WHERE
C.Id = #customerId AND
(c.code != 1 OR
EXISTS(
SELECT NULL
FROM Group G
JOIN GroupDetail D ON G.GroupId = D.DetailId
WHERE
C.CustomerId = G.CustomerId AND
G.Active = 1 AND
D.code = '1'
))