结果集的操作符

求并集：将两个结果集合并成一个结果集返回
union是求并集去重
union all是求并集不去重

select 1 as A from dual union select 1 as B from dual;
select 1 as A from dual union all select 1 as B from dual;

求交集：将两个结果集中公共部分作为结果集返回

select * from scott.emp where empno between 7369 and 7499 intersect select * from scott.emp where empno between 7499 and 7521;

求差集：定义操作符前面的结果集为A，后面的结果集为B，差集就是A结果集中减去A与B的交集，返回A中剩余的记录作为结果集。

select * from scott.emp where empno between 7369 and 7499 minus select * from scott.emp where empno between 7499 and 7521;

说明：
无论有多少个结果集，做运算的时候，相关结果集列的数量要一致。