Difference Between Primary Key and Unique Key in SQL

When designing a relational database, understanding the difference between a Primary Key and a Unique Key is crucial. Both are used to enforce data integrity but serve different purposes. Let’s explore their differences, importance, and key functionalities.

Primary key cannot have NULL value, the unique constraints can have NULL values.
There is only one primary key in a table, but there can be multiple unique constrains.

UNIQUE: This constraint when specified with a column, tells that all the values in the column must be unique. That is, the values in any row of a column must not be duplicate.
PRIMARY KEY: A primary key is a field which can uniquely identify each row in a table. And primary key constraint is used to specify a field in a table as primary key which is always contain unique data.

What is a Primary Key?

A Primary Key is a column or a set of columns in a database table that uniquely identifies each record. It ensures that no two rows have the same value and that the field cannot contain NULL values.

Key Features of a Primary Key:

• Uniqueness: Each value in the primary key column must be unique across the table.
• No NULL Values: A primary key column cannot have NULL values.
• Single Occurrence: A table can have only one primary key.
• Clustered Index: By default, a primary key creates a clustered index on the column.
• Auto Increment Support: The primary key can be set to auto-increment in many database systems.

What is a Unique Key?

A Unique Key is similar to a primary key in ensuring uniqueness but allows NULL values. It prevents duplicate entries in a specific column while allowing a single NULL value.

Key Features of a Unique Key:

• Uniqueness: Ensures all values in the column are unique.
• NULL Values Allowed: A unique key column can have only one NULL value.
• Multiple Occurrences: A table can have multiple unique keys.
• Non-Clustered Index: By default, a unique key creates a non-clustered index.
• No Auto Increment: A unique key does not support auto-incrementing values.

Primary Key vs. Unique Key: A Detailed Comparison

Importance of Primary Key and Unique Key in SQL

Why is a Primary Key Important?

• Ensures data integrity by preventing duplicate records.
• Serves as a reference point in foreign key relationships.
• Enables fast indexing and efficient searches.

Why is a Unique Key Important?

• Allows enforcing uniqueness without making it a primary identifier.
• Helps maintain data consistency by preventing duplicate values.
• Useful in scenarios where NULL values should be allowed.

FAQs on Primary Key vs. Unique Key

1. What is the key difference between a Primary Key and a Unique Key?

The primary key ensures uniqueness and does not allow NULL values, whereas a unique key allows only one NULL value and maintains uniqueness.

2. Can a table have multiple primary keys?

No, a table can have only one primary key.

3. How many unique keys can a table have?

A table can have multiple unique keys.

4. Can a Unique Key have NULL values?

Yes, but only one NULL value per unique column.

5. Can a Unique Key be a foreign key?

Yes, a unique key can be referenced as a foreign key in another table.

By understanding the differences between Primary Key and Unique Key, database designers can structure tables efficiently, ensuring data consistency and integrity.