SQL不是按表达式分组，查找已经参加过至少5门课程的学生

Question

select s.name, s.id
from student s join takes t on t.id = s.id
where s.name like 'D%' 
group by s.name, s.id
having (
    select count(distinct c.course_id)
    from course c
    where c.dept_name = 'History' and c.course_id = t.course_id)>4
order by s.name

我对GROUP BY如何运作感到困惑。我正在努力寻找从历史系开始至少学习5门课程且名字从D开始的学生。

对嵌套子查询不确定......

course(course id, title, dept name, credits)
student(ID, name, dept name, tot_cred)
takes(ID, course_id, sec_id, semester, year, grade)

Answer 1

您必须另外JOIN使用course表格：

select s.name, s.id
from student s 
inner join takes t on t.id = s.id
inner join course c on c.course_id = t.course_id
where s.name like 'D%' and c.dept_name = 'History'
group by s.name, s.id
having count(distinct c.course_id) >= 5

WHERE子句会返回名称以'D' 开始且的所有学生在历史系中选择至少一门课程。 HAVING子句会过滤掉历史系中任何不同课程的学生。