Chain/Thread Problem

Hi, 

I have a SQL problem for you all. What SQL qurey would I have to make
on the following table to extract the chain (thread) of emails in a
conversation.

        id|emailid|referenceid        -----------------------         1|  e125 |   Null             (Start of chain)
    2|  r1fe |   e125             (2nd letter in chain)         ...         ...         n|  e4ju |   r1fe
(3rdin chain)
 

ie
a sucessulf qurey would return rows 1,2 & n because they are all in
the chain.

Hope you can help me with my problem,