-- find all book ids coauthored by Smith and Jones:
-- (let's assume that the author_name is unique)
select book_id, count(*) from authors
where 
  author_name = 'Smith'
  or author_name = 'Jones'
group by book_id
having count(*) = 2

##</code><code>##

-- find all book ids coauthored by Smith and Jones:
-- (let's assume that the author_name is unique)
select book_id, count(*) from authors
where 
  author_name in ('Smith', 'Jones')
group by book_id
having count(*) = 2