Charles,
> > 3. All contigs where all clones have read = 'x'
SELECT * FROM contigs
WHERE NOT EXISTS ( SELECT contig_id
FROM clones WHERE clones.contig_id = contigs.contig_id
AND read <> 'x');
i.e. "Select all contigs not having any clone whose read is something
other than 'x' "
got it?
-Josh Berkus