集合运算符：UNION/UNION ALL 并集，INTERSECT 交集，MINUS 差集 
 
一、union求并集，公共部分只有包含一次 
例：求emp表ename中含’A‘或含有‘M’ 
SQL> select * from emp where ename like '%A%' 
  2  union 
  3  select * from emp where ename like '%M%'; 
 
二、union all求集并，公共部分只有包含二次 
例：求emp表ename中含’A‘或含有‘M’ 
SQL> select * from emp where ename like '%A%' 
  2  union all 
  3  select * from emp where ename like '%M%'; 
 
三、intersect求交集，只有包含公共部分 
例：求emp表ename中即含’A‘又含有‘M’ 
SQL> select * from emp where ename like '%A%' 
     intersect 
     select * from emp where ename like '%M%'; 
 
四、minus求差集，求集合A去除集合A和集合B的交集 
例：求emp表sal从700到1200 
SQL>select * from emp where sal between 700 and 1300 
    minus 
    select * from emp where sal between 1200 and 1400;

转：http://1632004.blog.163.com/blog/static/29991497201282653334529/