create table test2(st_id varchar2(6),arear number(10,2),tot_amt number(10,2));

insert into test2 values('1001',100,0);
insert into test2 values('1001',500,0);
insert into test2 values('1001',300,0);

insert into test2 values('1002',50,0);
insert into test2 values('1002',100,0);
insert into test2 values('1002',1000,0);
commit;


ST_ID----When-Validate-item--->TRIGGER

  begin
  select nvl(sum(arear),0) into :test2.arear from test2 where st_id=:test2.st_id ;
  end;