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

Try doing a join using conditions rather than a cross. The conditions would be sure that you match up the records correctly (you only include them if they are in both relations) rather than matching every record in the first relation to every record in the second relation.