--测试表A中的记录见下

select * from TestA;

--对两个结果集进行并集操作，不包括重复行

select * from TestA where id<3 union select * from TestA where id<6;

--对两个结果集进行并集操作，包括重复行行

select * from TestA where id<3 union all select * from TestA where id<6;

--对两个结果集进行交集操作，不包括重复行

select * from TestA where id<3 intersect select * from TestA where id<6;

--对两个结果集进行差操作（前面的结果集减去后面的），不包括重复行

select * from TestA where id<6 minus select * from TestA where id<3;

--对三个结果集进行取并集操作(取到的结果是从左到右依次的值不进行排序)在最后进行order by 操作

select * from (select * from TestA where id>5 union all select * from TestA where id<3 union all select * from TestA where id<2) order by id asc;

select * from (select * from TestA where id>5 union all select * from TestA where id<3 union all select * from TestA where id<2) order by id desc;