update customer c    
     set c.sal_amt = m.sal,
         c.mar_amt = m.mar,
         c.daily_amt = m.daily   
  from   
   customer c,   
   merchant m   
  where   
   c.c_id = m.t_id   
   and c.unit = m.unit     
   and c.band = m.band
   and c.loc = m.loc   
   and c.sid = :new_Id; --variable

 update customer c   
set (c.sal_amt,c.mar_amt,c.daily_amt) =
(select m.sal,m.mar,m.daily   
  from   
  merchant m   
  where   
   c.c_id = m.t_id   
   and c.unit = m.unit     
   and c.band = m.band
   and c.loc = m.loc)   
  where c.sid = :new_Id;