Find Records That Have Multiple Associated Records

A common type of table association in a relational database is a one-to-many relationship. For instance, a database representing a bookshelf may have an authors table where each record can be associated with multiple records in the books table. That relationship is represented by a author_id foreign key column on books that points to authors.id.

We can write a query to find all authors that have not zero or one, but multiple books by doing a join and then tacking on a having clause.

select authors.id, authors.name, count(books.id)
  from authors
  join books
    on authors.id = books.author_id
  group by authors.id
  having count(books.id) >= 2;

This will result in a listing of author ids, author names, and their number of books.

It does this by joining books to authors, grouping by the authors.id to produce a set of records unique to each author, and then combining multiple books by aggregating them with a count. The having clause is necessary because it is our way of filtering on an aggregate value, in this case the count.