sql select with exact outcome - sql

Question

Asking a simple question, just want everyone have fun to solve it.
I got 2 tables.
1. Student
2. Course
Student
+----+--------+
| id | name |
+----+--------+
| 1 | User1 |
| 2 | User2 |
+----+--------+
Course
+----+------------+------------+
| id | student_id | course_name|
+----+------------+------------+
| 1 | 1 | English |
| 2 | 1 | Chinese |
| 3 | 2 | English |
| 4 | 2 | Japanese |
+----+------------+------------+
I would like to get the result all student, who have taken English and Chinese, NOT English or Chinese.
Expected result:
+----+------------+------------+
| id | student_id | course_name|
+----+------------+------------+
| 1 | 1 | English |
| 2 | 1 | Chinese |
+----+------------+------------+
What we normally do is
select * from student join course on (student.id = course.student_id) WHERE course_name = 'English' OR course_name = 'Chinese'
but in this result I can get User2 record which is not my expected result. I want the record only display the User take the course English+Chinese only.

Answer 1

Just join onto course twice: once for English, once for Chinese. That is:
select student.*
from student
join course english_course on student.id = english_course.student_id
join course chinese_course on student.id = chinese_course.student_id
where english_course.course_name = 'English'
and chinese_course.course_name = 'Chinese'
or even
select * from student
where exists (select 1 from course
where course.course_name = 'English' and course.student_id = student.id)
and exists (select 1 from course
where course.course_name = 'Chinese' and course.student_id = student.id);
which will also eliminate duplicate (student_id,course_name) entries from course.
I'm assuming (student_id,course_name) is indexed to drive both these. Your naming is a bit odd: the "course" table doesn't describe a course, it describes the association from a student to a course. Personally, I'd call it "student_course" (or similar, maybe suffix "_map" or "_link") and have it contain a "course_id" referencing a course table with an id and name.
(I also prefer to have primary keys named consistently rather than calling them "id" in their own table, but that's just being picky, and much more subjective)
Just for fun:
select student.*
from student
join course on student.id = course.student_id
where course.course_name = 'English'
intersect
select student.*
from student
join course on student.id = course.student_id
where course.course_name = 'Chinese'
The reality is that using "intersect" to compare two result sets based on the same tables is a bit silly.

Answer 2

You can use a having clause to enforce to matches:
select s.student_id
from student s
join course c
on s.id = c.student_id
and c.course_name in ('English', 'Chinese')
group by
s.student_id
having count(distinct c.course_name) = 2
To retrieve other columns, you could join on this query:
select *
from student s
join course c
on s.id = c.student_id
join (
<query from above here>
) filter on s.id = filter.student_id

Answer 3

You can use the IN operator
select * from student join course on (student.id = course.student_id)
WHERE course_name = 'English' and student.id IN
(select student.id from student join course on (student.id = course.student_id)
WHERE course_name = 'Chinese')

Answer 4

You would probably want to do a self-join:
SELECT left.student_id sid1, right.student_id sid2,
left.course_name cn1, right.course_name cn2
FROM course left, course right
WHERE sid1 = sid2 AND
cn1 = 'English' AND cn2 = 'Chinese'
Joining that to the student table shouldn't be difficult.