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- an attribute used strictly for retrieval purposes (can be composite), for example: Phone number, Last Name and Phone number, etc.
- all other candidate keys not chosen as the primary key
- An attribute in one table that references the primary key of another table OR it can be null.
- Both foreign and primary keys must be of the same data type
- For example: Employee(EID, First Name, Last Name, SIN, Address, Phone, BirthDate, Salary, DepartmentID) -DepartmentID is the Foreign key. all other candidate keys not chosen as the primary key
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