What is a primary key vs unique key?

Primary Key vs. Unique Key in Databases: A Clear Explanation

I. Introduction

Relational databases use primary keys and unique keys to ensure data integrity. Many get confused between these two. This post aims to clearly explain their differences so you can design databases effectively.

II. What is a Primary Key?

A primary key uniquely identifies each row in a table. Think of it as a row's unique ID. It has two important characteristics:

• Uniqueness: No two rows can have the same primary key value.
• Non-null: A primary key value cannot be empty (NULL).

Example: In a "Customers" table, the customer ID might be the primary key. Each customer has a unique ID.

Primary keys are crucial for data integrity and linking tables together using foreign keys.

III. What is a Unique Key?

A unique key also ensures uniqueness—but for a column or set of columns. However, unlike a primary key, it allows NULL values (only one NULL value is permitted).

Example: In an "Employees" table, the email address could be a unique key. Employees have unique emails, but a new employee might not have one immediately, allowing for a NULL for a short time.

Unique keys are useful when you want to ensure uniqueness without the strict NOT NULL requirement of a primary key. They are sometimes used as natural keys, using existing meaningful attributes as identifiers instead of artificially generated IDs.

IV. Primary Key vs. Unique Key: A Comparison

Feature	Primary Key	Unique Key
Uniqueness	Yes	Yes
NULL Values Allowed	No	Yes (only one is permitted)
Number per Table	One	Multiple
Default Constraint	Enforced by the database	Can be enforced
Usage	Unique row identification, relationships	Unique constraints beyond the primary key

V. When to Use Which Key?

Choose a primary key when you need a guaranteed unique identifier, enforced by the system, for every row. This is the most common scenario. Use a unique key when you need a unique constraint, but allowing a NULL value is acceptable (e.g., email address until one is assigned).

VI. Conclusion

Primary keys guarantee uniqueness and non-null values. Unique keys guarantee uniqueness, but NULL is permitted. Understanding this difference is fundamental to solid database design. Practice using these concepts to improve your database skills!

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