CHAPTER 6 What are relationships?
Rules all around usRelationships tell the taleConnecting the dots
This chapter defines rules and relationships and the three different levels at which relationships can exist: conceptual, logical, and physical. Data rules are distinguished from action rules. Cardinality and labels are explained so that you can read any data model as easily as reading a book. Other types of relationships, such as recursive relationships and subtyping, are also discussed.
In its most general sense, a rule is an instruction about how to behave in a specific situation. The following are examples of rules that you are familiar with: