[zvmaz]

I tried to study C. J. Date's books to understand relational theory... suffice it to say that I got nothing from his books, except a deep irritation partly due to his absolute pedantry...

I finally learned SQL with a gentle introduction by Alan Beaulieu. I stumbled upon another book that's about the theory: Applied Mathematics for Database Professionals, by Lex deHaan, and Toon Koppelaars. Maybe these authors will benevolently teach me relational theory.

But please avoid C. J. Date's books. And don't be him when writing a book or trying to explain something to another human being.