# What are primary key constraints in relational databases?

Source: https://zilliz.com/ai-faq/what-are-primary-key-constraints-in-relational-databases
Author: zilliz

Primary key constraints are a fundamental concept in relational databases, serving as a way to uniquely identify each record within a table. A primary key is a specific column or a combination of columns that guarantees that no two rows in the table can have the same values for that key. This ensures data integrity by preventing duplicate entries, which is crucial for maintaining a reliable database. When a primary key is defined, the database enforces this uniqueness constraint, ensuring that operations on the data uphold this rule.

In practical terms, primary keys are typically implemented on columns that contain unique identifier values, such as user IDs, product codes, or order numbers. For example, if you have a table called `Users`, you might set the `user_id` column as the primary key. This means that each user in the table will have a distinct `user_id`, and attempts to insert a user with a duplicate `user_id` will result in an error. Additionally, primary keys may not contain NULL values, reinforcing the requirement that every record must have a unique identifier to access it reliably.

Moreover, primary keys often play a role in establishing relationships between tables. When one table references a primary key from another, it creates a foreign key relationship, which is essential for data normalization. For instance, if you have an `Orders` table that includes a `user_id` foreign key corresponding to the `Users` table's primary key, this relationship helps to maintain referential integrity. By checking that the `user_id` exists in the `Users` table, the database ensures that every order is tied to a valid user, preventing orphaned records. Overall, primary key constraints are vital for maintaining the structure and integrity of data in relational databases.