Relationship Attributes in ER Diagrams
The only kind of relationship that has its own key would be a many-to-many relationship. The key of this relationship is, naturally, the combination of foreign keys with each of its parents.
If there are other attributes of a relationship, then it is not a relationship at all, it is an entity in itself.
Keys associated with one-to-many (and one-to-one) relationships are found in the child table as foreign keys.
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