Question

I have a table as below which has a relationship between the requestID and PrimaryRequestID:

RequestID   PrimaryRequestID    Prefix  
-----------------------------------------------------------
1           NULL            MyPrefix
2           1               NULL

What I want to do is join this so I get the following output:

1   NULL    MyPrefix
2   1       MyPrefix

I’ve tried several joins, but I still end up with a NULL prefix.

Edit: This is probably as close as I’ve gotten:

SELECT  
    ra.RequestID 
    ,CASE WHEN rb.Prefix IS NULL THEN ra.Prefix ELSE ra.Prefix END AS Prefix
FROM requests ra
LEFT  JOIN requests rb on ra.RequestID = rb.PrimaryRequestID

Answer 1

Try:

select S.RequestId, S.PrimaryRequestID, coalesce(P.Prefix, S.Prefix) As Prefix
from myTable P
right join myTable S on P.RequestID = S.PrimaryRequestID