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

I figured out below based on wiki.

R:= \project_{pizzeria, pizza} (\select_{age>30} (Person \join Eats \join Serves))

S:= \project_{pizza} (\select_{age>30} (Person \join Eats \join Serves))

Final solution:

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

\diff

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