Computer Sciene of Udayana State University

April 18, 2009


Filed under: Computer Science — ignaga @ 11:38 AM

Super Key

A superkey is an attribute or a set of attributes that uniquely identify the relation. That is, no two tuples have the same values on the superkey. By definition, a relation consists of a set of distinct tuples. The set of all attributes in the relation forms a superkey.

Candidate Key

A key K is a minimal superkey, meaning that any proper subset of K is not a superkey. It is possible that a relation has several keys. In this case, each of the keys is called a candidate key.

Primary Key

The primary key is one of the candidate keys designated by the database designer. The primary key is often used to identify tuples in a relation.

The primary key constraint specifies that the primary key value of a tuple cannot be null and no two tuples in the relation can have the same value on the primary key. The DBMS enforces the primary key constraint. For example, if you attempt to insert a record with the same primary key as an existing record in the table, the DBMS would report an error and reject the operation.

Foreign Key

In a relational database, data are related. Tuples in a relation are related and tuples in different relations are related through their common attributes. Informally speaking, the common attributes are foreign keys. The foreign key constraints define the relationships among relations.

External Key

The external key is a lexical attribute (or compilation lexical attribute) that the values always identify an object instance.


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