I want to query for records containing multiple subjects. But because
the concatenated subject field can be very long, I have stored each
subject separately within an indexed wordstore (to speed up queries).
There is also a subject lookup table. For example, I want to return a
record that contains both 'Queen' and 'Mary' within subjects. If I was
just searching a concatenated subject field, then an AND would be fine.
What's the most efficient way to return only resource records with the
multiple subjects when querying the wordstore table?

I've tried using a Union All and then grouping by count to only show
duplicates, but that doesn't seem to work:

select r.resource_sn, r.concat_subjects from resources r
join subjectlookup sl on sl.resource_sn = r.resource_sn
join wordstore ws on ws.wordstore_sn = sl.wordstore_sn
where ws.text = 'Queen'
Union all
select r.resource_sn, r.concat_subjects from resources r
join subjectlookup sl on sl.resource_sn = r.resource_sn
join wordstore ws on ws.wordstore_sn = sl.wordstore_sn
where ws.text = 'Mary'
group by r.resource_sn, r.concat_subjects having count(*) >1