Relationships are a bit more complex than they might seem at first. They carry a lot of information. Some might say that they are the heart of the data model, since, to a great extent, they describe the rules of the business and the constraints on creating, modifying, and deleting instances. For example, you can use cardinality to define exactly how many instances are involved in both the child and parent entities in the relationship. You can further specify how you want to handle database actions such as INSERT, UPDATE, and DELETE using referential integrity rules.
Data modeling also supports highly complex relationship types that enable you to construct a logical model of your data that is understandable to both business and systems experts.
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