Primary Key vs. Unique Key in a Database

When it comes to database design, understanding the differences between a primary key and a unique key is crucial.

Primary Key

The primary key is a column or a group of columns that uniquely identify each row in a table. It must have a unique value for each record and cannot contain NULL values. The primary key is used to enforce entity integrity and ensure data consistency in the database.

Unique Key

A unique key is a constraint that ensures the values in a column or a group of columns are unique across the table. Unlike a primary key, a unique key can contain NULL values, allowing multiple rows to have NULL entries in the unique key column. However, only one row can have a NULL value in the unique key column for columns with a unique constraint.

Focus Keyword: Primary Key vs. Unique Key

In summary, the main difference between a primary key and a unique key lies in their uniqueness and handling of NULL values. While both enforce data integrity and uniqueness, a primary key uniquely identifies each row and does not allow NULL values, whereas a unique key ensures unique values but can allow NULL entries with restrictions.