SQL query - list of items in one table not in another

Question

I need some help with a SQL query. I have a table of courses and a table that contains user id and course id, denoting courses that user has taken (might not have taken any; no entry in that table for that user id).

I need a query to return the list of courses not taken.

Course Category table
    CategoryID
    Caegory

Courses table
    CourseID
    CategoryID
    CourseName
    ...

UserCourse table
    UserID
    CourseID

Answer 1

you can use not exists

Select * 
From Courses c 
Where Not Exists (Select 1 From UserCourse uc Where uc.CourseID = c.CourseID)

Answer 2

This will just list the course

select *
from Courses C
Left join CourseCategory cc on
cc.CategoryID = c.CategoryID
where CourseID not in (Select CourseID from UserCourse where UserID = 14)

Answer 3

what i need is for a given user id and course category, what courses within that category have not been taken by this user

(This should have been in the request by the way.)

So:

1. Select from courses .
2. Limit to the desired category.
3. Limit to courses not in the set of courses taken by the user.

The query:

select *
from courses
where categoryid = 123
and courseid not in (select courseid from usercourse where userid = 456);

Answer 4

Another way of writing same query, which will perform faster.

select C.CourseID,C.CategoryID
from Courses C
Left join CourseCategory cc on
cc.CategoryID = c.CategoryID
left join UserCourse  uc 
on C.CourseID=uc.CourseID
where uc.CourseID is null