In database design, a secondary key is utilized to retrieve and sort data in ways distinct from the primary key.

More specifically, a secondary key, also referred to as a non-primary key, comprises one or more attributes that can uniquely identify a record, albeit not as the primary key. While the primary key serves as the principal means of identifying records within a table, secondary keys offer alternative methods for accessing or retrieving those records. They are particularly advantageous in scenarios where the primary key is unknown or when there is a need to sort or retrieve data in a different sequence than that defined by the primary key.

For example, consider a database table containing student information where the primary key is the student ID. A secondary key could be the student’s email address or phone number. These secondary keys enable data retrieval using these attributes, even in cases where the student ID is not available.

Additionally, secondary keys can be employed to enforce specific constraints or rules on the data. For instance, a secondary key could be implemented to ensure that all email addresses within the student table remain unique. This constraint would prevent multiple students from sharing the same email address in the database.

Furthermore, secondary keys can be leveraged to create indexes on the database table. An index is a data structure that enhances the speed of data retrieval operations. By establishing an index on a secondary key, you can significantly improve the performance of queries that utilize this key. For instance, if you often need to locate students by their email address, creating an index on the email address field (a secondary key) would expedite these queries.

In conclusion, secondary keys in database design serve multiple purposes, including facilitating data retrieval, enforcing data constraints, and enhancing query performance. They provide both flexibility and efficiency in the management and access of data within a database.