select username from tab1
union
select username from tab2
union
select username from tab3
...
union
select username from tabn

select distinct username from (
select username from tab1
union all
select username from tab2
union all
select username from tab3
...
union all
select username from tabn
)

SQL> select  tbrq from dbscgs_mx
  2  union
  3  select  tbrq from dbscgs_mxbak
  4  union
  5  select  trunc(lrrq) from dbscgs_mx
  6  union
  7  select  trunc(lrrq) from dbscgs_mxbak;
......
已选择57行。

实际：187

select distinct rq from
(select tbrq rq from dbscgs_mx
union all
select tbrq rq from dbscgs_mxbak
union all
select trunc(lrrq) rq from dbscgs_mx
union all
select trunc(lrrq) rq from dbscgs_mxbak);
......

已选择57行。

实际：171