Alternate Key

An alternate key, also known as secondary key, is a candidate key that is not selected as the primary key for a database table. It is a unique identifier for each record in a table and is used to ensure data integrity and facilitate efficient retrieval of data. Unlike the primary key, which can be selected by the database designer, an alternate key is chosen based on specific business requirements.

Properties of Alternate Key
- Uniqueness: An alternate key must have unique values for each record in a table. This ensures that every record can be identified uniquely.
- Non-nullability: Alternate keys cannot have null values. Each record in a table must have a valid value for the alternate key.
- Candidate Key: An alternate key is a candidate key that is not chosen as the primary key. This means that it is a potential primary key for the table.

Use of Alternate Key
- Data Integrity: Alternate keys ensure that each record in a table is uniquely identified, preventing duplicates and maintaining data integrity.
- Indexing: Alternate keys can be used as indexes to facilitate faster retrieval of data from the table. By creating an index on the alternate key, the database can quickly locate the desired records.
- Foreign Key References: Alternate keys are often used as reference keys in other tables. They can be used as foreign keys to establish relationships between tables and maintain data consistency.

Example
Consider a database table called "Customers" with attributes such as customer_id, name, email, and phone number. In this case, the primary key could be customer_id, uniquely identifying each customer. However, email and phone number could serve as alternate keys, as they are also unique for each customer.

Conclusion
Alternate keys play a crucial role in database design by providing additional unique identifiers for records in a table. They help maintain data integrity, enable efficient data retrieval, and support relationships between tables. By carefully selecting alternate keys based on specific business requirements, database designers can create robust and effective database systems.