Can an Attribute Be a Primary and Foreign Key?
In Your Case, If There Is a One-to-One or a One-to-Zero-or-One Relationship Between User and Employee, Then Yes, the User_Id in the Employee Table Can Be...
In your case, if there is a one-to-one or a one-to-zero-or-one relationship between User and Employee , then yes, the User_ID in the Employee table can be Foreign Key (FK) and Primary Key (PK) at the same time.
Can same attribute be primary and foreign key?
Yes, it is legal to have a primary key being a foreign key. This is a rare construct, but it applies for: a 1:1 relation. The two tables cannot be merged in one because of different permissions and privileges only apply at table level (as of 2017, such a database would be odd).
Can any attribute be a foreign key?
Each foreign key is enforced independently by the database system. Therefore, cascading relationships between tables can be established using foreign keys. A foreign key is defined as an attribute or set of attributes in a relation whose values match a primary key in another relation.
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