Find the addresses and author IDs of the course authors that teach at least two courses.
create table courses (author_id varchar, PRIMARY KEY (author_id)); create table course_authors_and_tutors (address_line_1 varchar, author_id varchar, PRIMARY KEY (address_line_1))
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