Relationships affect the attributes of the entities which are involved in the relationship. When specifying a relationship between two entities, all of the key attributes of one entity may be transferred to the other entity. The transferred key attributes are referred to as either parent keys or join attributes. The following definitions apply:
A parent key is an attribute(s) which is present in a child entity as a result of a parent/child relationship.
Example: An Order Line entity has a parent key of order number, i.e. order number is an attribute of Order Line because of a parent/child relationship with the Order entity.
A join attribute is an attribute(s) which an entity contains as a result of a join relationship.
Example: An Order entity has a join attribute of customer number from the Customer entity.
The importance of the parent keys and join attributes is made clear when you consider the attribute rule.