List the author ids listed more than once in the courses
select author_id from courses group by author_id having count ( author_id ) >1
Find the addresses and author IDs of the course authors that teach at least two courses.
select t1.address_line_1, t2.author_id from course_authors_and_tutors as t1 join courses as t2 on t1.author_id = t2.author_id group by t2.author_id having count ( * ) > = 2
How about those who teaches at least one course?
select t1.address_line_1, t2.author_id from course_authors_and_tutors as t1 join courses as t2 on t1.author_id = t2.author_id group by t2.author_id having count ( * ) > = 1