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

Based on the assumption that all pizzerias serve at least one type of pizza, we will find that the group of pizzas that people over 30 do NOT EAT will be sold by all the pizzerias EXCEPT the one(s) who sell exclusively pizzas which people over 30 do EAT. Did it help?