How to find all pizzerias that serve every pizza eaten by people over 30?

Question

I\'m following the Stanford Database course and there\'s a question where we have Find all pizzerias that serve every pizza eaten by people over 30 using Re

Answer 1

Definitely this is the concept of division operator in relational algebra.

But I tried on that course. The RA Relational Algebra Syntax doesn't support dev operator. So I used diff and cross instead. Here is my solution:

\project_{pizzeria}(Serves)
\diff
\project_{pizzeria}(
    (\project_{pizzeria}(Serves) 
    \cross 
    \project_{pizza}(\project_{name}(\select_{age>30}(Person))\join Eats))
    \diff
    \project_{pizzeria,pizza}(Serves)
)