Sekundærnøkkel

Sekundærnøkkel is a term used in database management. It refers to one or more columns in a table that are not the primary key but are used to uniquely identify or efficiently retrieve rows. Unlike a primary key, which must be unique and non-null, a sekundærnøkkel can contain duplicate values and can be null. However, a sekundærnøkkel is often constrained to be unique to enforce data integrity if it represents a business rule, such as a unique email address for a user.

The primary purpose of a sekundærnøkkel is to improve query performance. By creating an index on the columns that constitute the sekundærnøkkel, the database system can quickly locate specific rows without having to scan the entire table. This is particularly beneficial for frequently searched columns that are not part of the primary key. For example, in a customer table, the primary key might be a customer ID, but a secondary key could be the customer's email address, allowing for fast lookups based on email.

While a primary key guarantees uniqueness across all rows, a sekundærnøkkel might enforce uniqueness through a unique index. This distinction is important. If a sekundærnøkkel is not marked as unique, it serves solely as an access path for faster data retrieval. If it is marked as unique, it acts as an additional identifier for records, similar to a primary key but without the primary key's role in table relationships. The use of secondary keys is a fundamental aspect of relational database design for optimization and data integrity.