Beginner...update stmt.

Hi, I was wondering if someone might critique this statement (see below). I wouldn't expect that it should take over 5 hrs to run and still not complete executing???? Any assistance you can offer I will greatly appreciate it. Thanks. ---FYI--- I'm expecting to update about 3 million rows. The idea is to get the distinct col(s) from tables Step1 and upc_rslts_fcst and use them against compare_forecast (52 recs for each upc...) UPDATE compare_forecast SET compare_forecast.New_A12 = (selectStep1.sumofregunits * upc_rslts_fcst.New_R_PCT from upc_rslts_fcst, Step1 WHERE compare_forecast.upc = Step1.upc and upc_rslts_fcst.upc = Step1.upc and compare_forecast.district = upc_rslts_fcst.district and compare_forecast.location = Step1.location and compare_forecast.week = Step1.week and Step1.week = upc_rslts_fcst. week );