oracle查詢兩個表相同字段的區別

select t1.tac from
(select tac from uadpt_tac
minus
select tac from uadpt_tac_act_data) t1
union 
select t2.tac from
(select tac from uadpt_tac_act_data
minus
select tac from uadpt_tac) t2;