如何检索在oracle中发生两次以上的记录？

Question

我有这张桌子

create table student (
   stu_id int,
   s_name nvarchar(max),
   s_subject nvarchar(max),
)

这是数据

insert into student values(123,'pammy','English');
insert into student values(123,'pammy','Maths');
insert into student values(123,'pammy','Chemistry');
insert into student values(124,'watts','Biology');
insert into student values(125,'Tom','Physics');
insert into student values(125,'Tom','Computer';
insert into student values(125,'Tom','ED';

所以我想检索已发生两次以上的记录.我的代码是

select stu_id,s_Name 
from student 
group by stu_id,s_Name 
having count(stu_id) >2 ;

结果很完美.

但是当我想要s_subject它时也没有选择任何行.我不知道为什么.

select stu_id,s_Name,s_subject 
from student 
group by stu_id,s_Name,s_subject 
having count(stu_id) >2 ;

Answer 1

这是因为没有一个学生每个科目有多个记录.

select stu_id,s_Name,s_subject 
from student 
group by stu_id,s_Name,s_subject 
having count(stu_id) >2 ;

此代码要求出现两次以上具有相同学生ID,姓名和主题的记录.您的样本中的所有记录都不符合此要求.

但是,如果您真正想要的是任何学生参加两个以上课程的ID,姓名和科目,这可以很容易地完成.

使用稍微修改过的初始SQL版本作为过滤器,我们得到:

select stu_id, name, subject
from student
where stu_id in (   select stu_id 
                    from student 
                    group by stu_id 
                    having count(stu_id) >2 );

希望这可以帮助.