SELECT plangroup.kodgp kgp, plangroup.naim nim, cena_gp.cg_cena cgc,
(case when a.oetrn_invdate<'13.05.2008'  then  
 case when a.oetrn_price> 2000  then
  round(sum(a.oetrn_oinvqty*a.oetrn_price)/sum(a.oetrn_oinvqty), 2 ) else
  round(sum(a.oetrn_oinvqty*a1.oemer_pricevlt)/sum(a.oetrn_oinvqty), 2 ) end
  else round(sum(a.oetrn_oinvqty*a1.oemer_pricevlt)/sum(a.oetrn_oinvqty), 2 ) end) price3,
(case when a.oetrn_invdate<'13.05.2008'  then  
 case when a.oetrn_price> 2000  then
  round(sum(b.oetrn_oinvqty*b.oetrn_price)/sum(b.oetrn_oinvqty), 2 ) else
  round(sum(b.oetrn_oinvqty*b1.oemer_pricevlt)/sum(b.oetrn_oinvqty), 2 ) end 
  else round(sum(b.oetrn_oinvqty*b1.oemer_pricevlt)/sum(b.oetrn_oinvqty), 2 ) end) price4
FROM plangroup
join cena_gp on plangroup.item=cena_gp.cg_item and
cena_gp.cg_data='01.05.2008'
left join oetrn as a ON plangroup.item=a.oetrn_item and
(a.oetrn_invdate BETWEEN '02.05.2008' and '01.06.2008')
left join oemer as a1 ON
a.oetrn_ordnum=a1.oemer_ordnum and
(a.oetrn_linnum=a1.oemer_linnum)
left join oetrn as b ON plangroup.item=b.oetrn_item and
(b.oetrn_invdate BETWEEN '02.05.2008' and '01.06.2008')
left join oemer as b1 ON
b.oetrn_ordnum=b1.oemer_ordnum and
(b.oetrn_linnum=b1.oemer_linnum)
WHERE
plangroup.prog='цены'
group by  1 , 2 , 3 
order by  1