Question

I have a query to join 2 tables in the following way.

Table1 has a field (LinkTo) that may (or may not) appear in one of three fields in table Table2 (Link, Link1, Link2). I need try joining LinkTo to Link, then Link1 then Link2. As soon as I match on one (say Link1) I should stop trying to join.

Thanks

Mark

Answer 1

This might be what you are looking for. It joins in the order of fields you mentioned (edited to answer a question from the comments):

SELECT t1.*, t2.*,
    CASE WHEN (t1.LINKTO = t2.LINK) THEN 'LinkedWithLink' 
         WHEN (t1.LINKTO <> t2.LINK 
           AND t1.LINKTO = t2.LINK1) THEN 'LinkedWithLink1' 
         WHEN (t1.LINKTO <> t2.LINK1 
           AND t1.LINKTO <> t2.LINK1 
           AND t1.LINKTO = t2.LINK2) THEN 'LinkedWithLink2' END AS chosen_join
FROM TABLE1 t1 
[LEFT OUTER] JOIN TABLE2 t2
ON (t1.LINKTO = t2.LINK OR 
   (t1.LINKTO <> t2.LINK AND t1.LINKTO = t2.LINK1) OR
   (t1.LINKTO <> t2.LINK1 AND t1.LINKTO <> t2.LINK1 AND t1.LINKTO = t2.LINK2)

Note, I don’t know whether you need a LEFT OUTER join or not… In any case, you should be aware of a potentially very slow execution plan with this…