Primary Keys vs. Unique Keys in SQL Server

Primary keys and unique keys are both important in SQL Server database design, but they serve slightly different purposes.

Primary Keys

A primary key is a column or a set of columns that uniquely identifies each row in a table. It ensures that each record in the table has a unique identifier. Primary keys also enforce entity integrity and are used as foreign keys in relationships with other tables. In SQL Server, only one primary key constraint can be defined per table.

Unique Keys

A unique key is a constraint that ensures all values in a column or a set of columns are unique. Unlike a primary key, a unique key allows for one or more NULL values. Unique keys are used to enforce data integrity and avoid duplicate values in specific columns. In SQL Server, multiple unique key constraints can be defined per table.

In summary, while primary keys uniquely identify each row in a table and are crucial for data integrity and relationships, unique keys ensure that specific columns have unique values and prevent duplicates.