select max(id) from yourtable where sts=0 and id not in (select ref_id from yourtable);
Isn't it?
The OP will need to explain further as we are all guessing. As I mentioned in my earlier (accidental top - curses GMail) post, table structures and the query or queries that don't work would be useful. So would a description of the problem that is being solved since there could be better approaches.
Maybe we mis-interpreted but as written this is the solution. My out-loud thinking was a more verbose version of this.