primærnøkler

A primary key is a column or a set of columns in a database table that uniquely identifies each row in that table. It ensures that the data in the table is unique and not duplicated. Primary keys are essential for maintaining the integrity of the data and for establishing relationships between tables in a relational database.

Primary keys have several key characteristics:

1. Uniqueness: Each value in the primary key column must be unique. No two rows can have the same primary key value.

2. Non-nullability: Primary key columns cannot contain null values. Every row must have a valid primary key value.

3. Immutability: The value of a primary key should not change once it is set. This ensures that the uniqueness constraint is maintained.

Primary keys are often used as foreign keys in other tables to establish relationships between tables. For example, in a database with tables for customers and orders, the customer ID in the orders table would be a foreign key that references the primary key in the customers table.

Primary keys can be of different types, including:

1. Simple primary key: A single column that uniquely identifies each row.

2. Composite primary key: A combination of two or more columns that together uniquely identify each row.

Primary keys play a crucial role in database design and management, helping to ensure data integrity, optimize query performance, and facilitate data relationships.