ลองแบบนี้ดูครับ
select p.pttype,p.name,count(dt.vn) as cnt_vn,count(distinct dt.hn) as cnt_hn
from dtmain dt
left outer join vn_stat vn on vn.vn = dt.vn
left outer join pttype p on p.pttype = vn.pttype
where dt.vstdate between "2007-09-01" and "2007-09-31" and dt.vstage in ("13","14")
group by p.pttype,p.name
cnt_hn = คน
cnt_vn = ครั้ง