SELECT d.KodeBrg,d.NamaBrg,sum((Harga/konv)*(Jml*konv))/sum(jml*konv) HargaAvg,SUBDATE(x.tglbeli,INTERVAL 1 MONTH) TglAwal,m.TglBeli TglAkhir 
FROM tr_beli_d d JOIN tr_beli_m m ON d.NoBeli=m.NoBeli 
LEFT JOIN (SELECT KodeBrg,MAX(m.TglBeli) tglbeli FROM tr_beli_d d 
JOIN tr_beli_m m ON d.NoBeli=m.NoBeli GROUP BY d.KodeBrg) x ON d.KodeBrg=x.kodebrg
WHERE m.tglbeli >= SUBDATE(x.tglbeli,INTERVAL 1 MONTH)
AND d.kodebrg=:KodeBrg
GROUP BY d.kodebrg