What is the difference between a primary key and a unique key?

Primary Key vs Unique Key: Understand the Difference

One common question that often comes up in Database Administrator interviews is the distinction between a primary key and a unique key. While both serve to enforce data integrity within a database, there are key differences between the two.

Primary Key

A primary key is a column or a set of columns that uniquely identifies each record in a table. It must contain unique values and cannot be null. Additionally, there can be only one primary key in a table. The primary key is crucial for maintaining data integrity and is typically used as a reference point for foreign keys in related tables.

Unique Key

A unique key, on the other hand, also enforces uniqueness within a column or a set of columns. However, unlike a primary key, a unique key allows one null value. Additionally, multiple unique keys can exist within a table, offering flexibility in enforcing data integrity constraints.

In summary, while both primary key and unique key constrain the values in a column or set of columns to be unique, the primary key uniquely identifies each record in a table and does not allow null values, whereas a unique key allows for one null value and supports multiple instances within a table.

Understanding the distinction between primary key and unique key is essential for designing efficient and effective database structures. Incorporating these concepts into database design ensures data integrity and optimal performance.