The following is the most important rule to remember when creating a logical data model:
An attribute may be associated with, or attached to, only one entity in your data model.
If the same attribute is required in another entity, it must be the result of a relationship.
An attribute such as customer number may be associated with the Customer entity. Once the customer number attribute is attached to the Customer entity, it cannot be attached to any other entity in your data model. The customer number cannot be made an attribute of the Order or Invoice entities. If the Order entity requires a customer number to identify the customer which placed the order, than a relationship must be created between Order and Customer.
The Attribute Rule is very important for two reasons:
The Attribute Rule helps to create a logical data model instead of a physical model of files in the database. An attribute such as customer number cannot be added to more than one entity without using relationships to define the structure of the data.
It is important to note the difference between assigning the same attribute to more than one entity and having many similar attributes as part of many entities. You must decide if an attribute is actually related data from another entity or a separate piece of data.