咱们在编写sql语句时，可能会遇到minus、intersect 、union 的应用疑问，现举个简单例子说明下：
--minus 在前不在后select *
from (select 1
from dual
union all
select 2 from dual)
minus (select 2
from dual
union all
select 3 from dual);
--intersect前后交集
select *
from (select 1
from dual
union all
select 2 from dual)
intersect (select 2
from dual
union all
select 3 from dual)
select *
from (select 1
from dual
union all
select 2 from dual)
union (select 2
from dual
union all
select 3 from dual)
--union all 并集,不检查重复行
select *
from (select 1
from dual
union all
select 2 from dual)
union all (select 2
from dual
union all
select 3 from dual)