操作两组查询结果，进行交集、并集、减集运算
1、 union和union all进行并集运算
--union 并集、不重复
select id, name from student where name like 'ja%'
union
select id, name from student where id = 4;
--并集、重复
select * from student where name like 'ja%'
union all
select * from student;
2、 intersect进行交集运算
--交集（相同部分）
select * from student where name like 'ja%'
intersect
select * from student;
3、 except进行减集运算
--减集（除相同部分）
select * from student where name like 'ja%'
except
select * from student where name like 'jas%';
详细出处参考：http://www.jb51.net/article/27717.htm
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