ORACLE去重

1. distinct,不推荐这种去重方法，数据量大的时候执行效率很低。

select distinct sname 
  from student;

2. group by

select sname 
  from student
 group by sname;

3. rowid(伪列去重)

select sname
  from student t1
 where t1.rowid in(select min(rowid)
                     from student t2
                    where t1.sname= t2.sname
                  );

4. row_number() over()开窗函数

select sname
  from (select sname
               ,row_number() over(partition by sname) as rm
          from student
        ) t 
 where t.rm = 1;