CONNECTIONS Three Types of Relationships Between Entities 1 : 1 1 : N N : M The Player and Their Progress The Player and His Purchases Players and Matches PK = FK in Progress: no more than one row per player. It is not guaranteed that every player will have a row. One player has many purchases, and each purchase has one player. The key is placed on the “many” side. The player has been in many games, and there are many players in a game. It isn't stated directly. For each side: How many related rows are allowed, and are they required? Lack of a relationship: NULL in the foreign key or no child row.