select count(ov.hn) as chn 
from vn_stat ov, ovst ovst, patient pt 
where  ov.vn=ovst.vn and pt.hn=ov.hn and ov.vstdate between "2008-01-01" and  "2008-02-29"
 and ov.age_y>= 0 
 and ov.age_y<= 200 
 and ( (ov.pdx = "N189") 
 or (ov.dx0 = "N189") 
 or (ov.dx1 = "N189") 
 or (ov.dx2 = "N189") 
 or (ov.dx3 = "N189") 
 or (ov.dx4 = "N189") 
 or (ov.dx5 = "N189") )