case when then else end

 SELECT a.lim_code
      ,a.lim_bk_code
      ,d.bk_name
      ,a.lim_bt_code
      ,c.bt_name
      ,b.blood_sum
      ,b.bag_sum
      ,a.lim_bu_code
      ,e.bu_name
      ,a.lim_validday
      ,a.lim_value1
      ,a.lim_value2
      ,a.lim_value3
      ,a.lim_value4
      ,(case when b.blood_sum<=a.lim_value4 then 4
                       when b.blood_sum<=a.lim_value3 then 3
                       when b.blood_sum<=a.lim_value2 then 2
                       when b.blood_sum<=a.lim_value1 then 1
                       else 0 end) as lim_flag     
  FROM bd_dict_stock_limit as a
    left join bd_dict_type as c on a.lim_bt_code=c.bt_code
    left join bd_dict_kind as d on a.lim_bk_code=d.bk_code
    left join bd_dict_unit as e on a.lim_bu_code=e.bu_code
    left join (SELECT bs_bk_code,bs_bt_code,SUM(isnull(bs_num,0)) as blood_sum
                ,COUNT(bs_sn) as bag_sum from bd_stock 
                where bs_flag>=1 and bs_flag<6 
                group by bs_bk_code,bs_bt_code)
       as b on a.lim_bk_code=b.bs_bk_code and a.lim_bt_code=b.bs_bt_code
  where a.del_flag=0 and (b.blood_sum<=a.lim_value1 or b.blood_sum<=a.lim_value2 or b.blood_sum<=a.lim_value3 or b.blood_sum<=a.lim_value4)