A sample pokemon teaching database
The relationship between the Pokemon table and the Move table is inherently many-to-many (M:N): one pokemon can learn many moves, and one move can be learned by many pokemon. To account for this, we added a linking table called LearnHistory. Both the Pokemon table and the Move table have a one-to-many relationship (1:M) with LearnHistory, but are not related to each other. LearnHistory consists of two foreign keys: PokeID (the primary key of the Pokemon table), and MoveID (the primary key of the Move table).
The manner of participation between the Pokemon and Move tables is optional, both ways. A specific pokemon does not need to have learned a move from the move tutor, and a specific move does not need to have been taught at least once to be in the database.
The degree of participation is 0 or more (0:N) in both directions. A pokemon can learn any number of moves from the move tutor, since there is no upper limit. And since the relationship is optional, the lower limit is 0. Similarly, a move can be learned by any number of pokemon, and does not have to be learned by any pokemon to be in the database, making the degree also 0:N.
We are using the “DENY” deletion rule for both pokemon and moves. Doing this will allow us to mark pokemon as inactive while also keeping track of how popular a move is (rather than deleting the record of the pokemon ever learning it). Marking a move as inactive will also let us keep track of how many times a pokemon has been taught a move.