can you please tell me if Alice is friends with Bob?
SELECT T1.name FROM Person AS T1 JOIN PersonFriend AS T2 ON T1.name   =   T2.name WHERE T2.friend   =   'Bob'
Alice being friends with Bob implies checking if Alice's name appears in a table where Bob is listed as a friend; relationship between persons and their friends can be determined by joining a person table with a friend mapping table;