Types of Keys in DBMS
In the Relational Database Model, keys are important because they are used to ensure that each row in a table is uniquely identifiable. They are also used to establish relationships among tables and to ensure the integrity of the data.
Functional dependence:
The attribute B is functionally dependent on the attribute A if each value in column A determines one and only one value in column B.
Composite key:
A key may be composed of more than one attribute. Such a multi attribute key is known as a composite key.
Relational Database Keys:
Superkey: An attribute (or a combination of attributes) that uniquely identifies each row in a table.
Candidate key: A minimal (irreducible) superkey. A superkey that does not contain a subset of attributes that is itself a superkey.
Primary key: A candidate key selected to uniquely identify all other attribute values in any given row. Cannot contain null entries.
Secondary key: An attribute (or combination of attributes) used strictly for data retrieval purposes.
Foreign key: An attribute (or combination of attributes) in one table whose values must either match the primary key in another table or be null.
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Codd’s Relational Database Rules
