1，group by方法
查数据:
select count(num), max(name) from student --列出重复的记录数，并列出他的name属性
group by num
having count(num) >1 --按num分组后找出表中num列重复，即出现次数大于一次
group by num
having count(num) >1 --按num分组后找出表中num列重复，即出现次数大于一次
删数据：
1 delete from student
2 group by num
3 having count(num) >1
2 group by num
3 having count(num) >1
这样的话就把所有重复的都删除了。
2，用distinct方法 -对于小的表比较有用
1 create table table_new as select distinct * from table1 minux
2 truncate table table1;
3 insert into table1 select * from table_new;
2 truncate table table1;
3 insert into table1 select * from table_new;