从 INNER JOIN 查询返回元组的逆

Question

I am trying to get the remaining tuples from a query. In this case, course_id 3 and 4 from courses because the user admin@ has NOT taken these (they've only taken 1 and 2). The tables are already joined nicely and queries are working when I try a LEFT JOIN...

select course_name from courses LEFT OUTER JOIN users_courses ON users_courses.user_course_id = courses.course_id where users_courses.user_email = "admin@---.com";

But this returns the courses 1 and 2, not 3 and 4

Answer 1

you could do another thing by getting all of the courses that is not exists in the courses list for this user like this

select course_name from courses where course_id not in (select user_course_id from users_courses where user_email = 'admin@....')

update

forget to add the column name course_id

Answer 2

Ah. I misunderstood what you were after, Joseph's method will work, but, as a rule. you should try to avoid "in(select...".

To knock-out the courses the admin has taken, just add a null condition:

select course_name from courses LEFT OUTER JOIN users_courses
ON users_courses.user_course_id = courses.course_id
and users_courses.user_email = "admin@---.com"
where users_courses.course_id is null;