从两列映射中分组多对多关系

I have a SQL table that maps, say, authors and books. I would like to group linked authors and books (books written by the same author, and authors who co-wrote a book) together and ascertain how big these groups get. For example, if J.K. Rowling co-wrote with Junot Diaz, and Junot Diaz co-wrote a book with Zadie Smith, then I would want all three authors in the same group.I have a SQL table that maps, say, authors and