There are three main kinds of relationships, and it is important to determine both directions of the relationship when modeling the relationships between entities:
- 1-to-1: Both sides of the relationship have a degree of one and only one. (1-to-1 relationships are rare.)
- 1-to-M: One of the sides has a degree of one or many and the other side has a degree of one and only one.
- M-to-M: Both sides of the relationship are one or many relationships.