SELECT * from `donor_visits`
join(
select donor_visits.donor_id
from donor_visits
group by donor_visits.donor_id
having count(*) > 1) s on donor_visits.donor_id = s.donor_id
join donors
on donors.id = donor_visits.donor_id
where study_id = 21
and donor_visits.donor_id = 166
order by donor_visits.donor_id, visit_id