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 Relational Algebra only.
The problem consist of a small database with four relations:
Person(name, age, gender) // name is a key
Frequents(name, pizzeria) // [name,pizzeria] is a key
Eats(name, pizza) // [name,pizza] is a key
Serves(pizzeria, pizza, price) // [pizzeria,pizza] is a key
I know how to find which pizza's people over 30 eat and make a cross-product of them, so I could check which pizzeria has both.
I can make a list of all the pizzeria's that serve those pizza's, but I have no idea how to remove any pizzeria that only have one combination (like Dominos).
Chicago Pizza cheese cheese
Chicago Pizza cheese supreme
Chicago Pizza supreme cheese
Chicago Pizza supreme supreme
Dominos cheese cheese
Dominos cheese supreme
The Q&A forums tell us to use division and point us to several presentations. While I get what the result of the action would be, I don't really understand how to translate the formula's into relational algebra syntax.
Could anyone explain me what I'm missing, hopefully without giving the solution outright?
The solution is the join div operator http://en.wikipedia.org/wiki/Relational_algebra#Division_.28.C3.B7.29
See http://oracletoday.blogspot.com/2008/04/relational-algebra-division-in-sql.html