select max(id) from yourtable where sts=0 and id not in (select ref_id from yourtable);
select max(id) from yourtable where sts=0 and id not in (select ref_id from yourtable);
do note, this is whats known as an 'anti-join', and these can be pretty expensive on large tables.
+1
Though I suspect that with a partial index on (id, sts=0) and (ref_id, ref_id IS NOT NULL), though highly sensitive to density, that even for large total row counts it would perform pretty well; but I'm not knowledgeable in how smart we are here. Selecting, in descending order, (id where sts = 0), from the index and then poking into index(ref_id) should, particularly if the cross-set is sparse, pretty quickly find a non-match.
