Product tables are missing too. Let's say you have tables `foo` and `bar`. For every `foo` and every `bar`, you want the user to specify a `qux` value. Presently, what you need to do is:

(0) Create a table `foo_bar`, with fields `foo_id`, `bar_id` and `qux`. In particular, `qux` must be nullable. [Yuck!]

(1) Add triggers to `foo` and `bar` that automatically insert or delete rows from `foo_bar`.

(2) Hope [I'm not joking] the user remembers to set all the `qux` values in `foo_bar` whenever he inserts a row into either `foo` or `bar`.