In its most general sense, a rule is an instruction about how to behave in a specific situation. The following are familiar examples of rules:
· You must clean your room before you can go outside and play.
· If you get three strikes, you are out and it is the next batter’s turn.
· The speed limit is 55 miles per hour.
Many such rules can be visually captured on our data model through relationships. A relationship is displayed as a line connecting two entities, capturing the rule between these two entities. If the two entities are Employee and Department, the relationship can capture the rules “Each Employee must work for one Department” and “Each Department may contain one or many Employees.”