A Foreign Key consists of a collection of columns (UML Attributes) that together have some operational meaning - they enforce a relationship to access data in a related Table via its Primary Key.
Foreign keys are modeled in Enterprise Architect as operations with the stereotype FK. The operation's parameters are the columns involved in the key.
It is not necessary to define a Foreign Key just to access another Table through its Primary Key. Foreign Keys are a feature of some database management systems, providing 'extras' such as referential integrity checking that prevents the deletion of a record if its Primary Key value exists in some other Table's Foreign Key. The same thing can be achieved programmatically.
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