So if I've got a table of books, with columns "id," "title," and "language." I'm using the 2-letter ISO language codes, like en, fr, de, jp, etc. This works well (although it's not normalized) for books that are written in a single language. Currently, book.language is a varchar(10) which is long enough to hold up to 4 languages separated by spaces. But this is bad design. So what do I do if a book is written in more than one language? I can create a "language" table with all languages in it, and set up book.language as a foreign key to language.id or whatever. But how can I assign multiple languages to a book, without creating multiple book.language columns, unless that's what I have to do?