差集与交集

检验minus差集作用：(输出1)
select a from
(select 1 a from dual union all 
select 1 a from dual union all
select 3 a from dual union all 
select 4 a from dual ) t1
minus
select a from
(select 21 a from dual union all 
select 21 a from dual union all
select 3 a from dual union all 
select 4 a from dual ) t2;


检验intersect交集作用：(输出3,4)
select a from
(select 1 a from dual union all 
select 3 a from dual union all
select 3 a from dual union all 
select 4 a from dual ) t1
intersect
select a from
(select 21 a from dual union all 
select 3 a from dual union all
select 3 a from dual union all 

select 4 a from dual ) t2;

注:两者对结果集进行统计的结果，故有此也可得知，需要谨慎使用，效率肯定远低于union all