MySQL where joined value is multiple ANDs - mysql

Question

Running into a seemingly simple JOIN problems here..
I have two tables, users and courses
| users.id | users.name |
| 1 | Joe |
| 2 | Mary |
| 3 | Mark |
| courses.id | courses.name |
| 1 | History |
| 2 | Math |
| 3 | Science |
| 4 | English |
and another table that joins the two:
| users_id | courses_id |
| 1 | 1 |
| 1 | 2 |
| 1 | 3 |
| 2 | 1 |
I'm trying to find distinct user names who are in course 1 and course 2
It's possible a user is in other courses, too, but I only care that they're in 1 and 2 at a minimum
SELECT DISTINCT(users.name)
FROM users_courses
LEFT JOIN users ON users_courses.users_id = users.id
LEFT JOIN courses ON users_courses.courses_id = courses.id
WHERE courses.name = "History" AND courses.name = "Math"
AND courses.name NOT IN ("English")
I understand why this is returning an empty set (since no single joined row has History and Math - it only has one value per row.
How can I structure the query so that it returns "Joe" because he is in both courses?
Update - I'm hoping to avoid hard-coding the expected total count of courses for a given user, since they might be in other courses my search does not care about.

Answer 1

Join users to a query that returns the user ids that are in both courses:
select u.name
from users u
inner join (
select users_id
from users_courses
where courses_id in (1, 2)
group by users_id
having count(distinct courses_id) = 2
) c on c.users_id = u.id
You can omit distinct from the condition:
count(distinct courses_id) = 2
if there are no duplicates in users_courses.
See the demo.
If you want to search by course names and not ids:
select u.name
from users u
inner join (
select uc.users_id
from users_courses uc inner join courses c
on c.id = uc.courses_id
where c.name in ('History', 'Math')
group by uc.users_id
having count(distinct c.id) = 2
) c on c.users_id = u.id
See the demo.
Results:
| name |
| ---- |
| Joe |

Answer 2

You can use in operator and use select to generate list of potential users_id attending the second course, to find matching ones in the first course. This is many times faster than using joins.
select distinct u.users_id, users.name
from users_courses u, users
where u.users_id in (select distinct users_id from users_courses where courses_id = 2)
and u.courses_id = 1
and users.users_id = u.users_id

Answer 3

Almost similar to what #Nae's solution.
select u.name from users u
where exists
(select 1
from users_courses uc
where uc.course_id in (1, 2)
and uc.user_id = u.id
group by uc.user_id
having count(0) = 2);

Answer 4

Your code is close. Just use GROUP BY and a HAVING clause:
SELECT u.name
FROM users_courses uc JOIN
users u
ON uc.users_id = u.id JOIN
courses c
ON uc.courses_id = c.id
WHERE c.name IN ('History', 'Math')
GROUP BY u.name
HAVING COUNT(DISTINCT c.name) = 2;
Notes:
This assumes that users cannot have the same name. You might want to use GROUP BY u.id, u.name to ensure that you are counting individual users.
If users cannot take the same course multiple times, then use COUNT(*) = 2 rather than COUNT(DISTINCT).

Answer 5

I'd write:
SELECT MAX(u.name)
FROM users_courses uc
LEFT JOIN users u ON uc.users_id = u.id
WHERE uc.courses_id IN (1, 2)
GROUP BY uc.users_id
HAVING COUNT(0) = 2
;
For more complex conditions (for example requiring the user to be in certain classes but also not in certain classes such as "Science") this should also work:
SELECT MAX(u.name)
FROM users_courses uc
LEFT JOIN users u ON uc.users_id = u.id
GROUP BY uc.users_id
HAVING (
SUM(uc.courses_id = 1) = 1
-- user enrolled exactly once in the course 2
AND SUM(uc.courses_id = 2) = 1
-- user enrolled in course 3, 0 times
AND SUM(uc.courses_id = 3) = 0
)
;