Is there a canonical definition for a database schema?

I do not know how to say, I'm not an expert on the subject and I do not circle academic circles that use this kind of information better, but I'd start by Wikipedia . Not that she's always reliable, but it's always a good start to finding what she wants. Once confirmed, it may be that the information there is briefly canonical.

You are right that every SGDB uses the way it interprets, so it is that thing, there may even be something canonical, but what good is it if this form is not followed in practice? So in practice the definition of each became more important. For those who want to be a database specialist should know the canon well as well, although for interpretation, this may not be exactly possible. Having your own well-grounded interpretation may even be more important than having something canonical.