Types of Database Keys
- 2. Primary Key – This is an attribute or set of attributes that uniquely indentify a specific instance of a record or an entity. Every entity in the database must have a primary key whose value uniquely identifies instances or records of the entity. Features of the primary key are: - It must have a non – null value for each instance - The value must be unique - The value must not change or become null during the life of each entity instance
- 3. Candidate Key – In some instances, an entity will have more than one attribute that can serve as a primary key. Any key or minimum set of keys that could be a primary key is called a candidate key. Choose the identifier most commonly used by the user as long as it conforms to the properties of the Primary Key. Alternate Key (Secondary Key) – The Candidate Keys which are not chosen as the Primary Key are known as alternate keys. Composite Key – Sometimes it requires more than one attribute to uniquely identify an entity. A primary that is made up of more than one attribute is known as a composite key.
- 4. Foreign Key – A foreign key is an attribute within a child entity that completes a relationship by identifying the parent entity. Foreign keys provide a method for maintaining integrity in the data (called referential integrity) and for navigating between instances of an entity. Every relationship in the model must be supported by a foreign key.
- 5. TRN STUDENT ID LAST NAME FIRST NAME COURSE ID 100-000-001 SH-1001 ALLEN ROSENNA CHM-101 100-000-002 SH-1002 ALLWOOD PAULA-ANN BIO-101 100-000-003 SH-1003 BRADY BRIANA CHM-101 100-000-004 SH-1004 BURKE TIANA ICT-101