Difference Between Primary Key & Unique Key

Primary Key vs. Unique Key: Understanding the Core Differences in Database Design

Choosing the right constraints for your database tables is crucial for data integrity and efficiency. Two key constraints often cause confusion: the primary key and the unique key. While both ensure uniqueness, they differ significantly in their purpose and implications. This article clarifies the distinctions between primary keys and unique keys, helping you make informed decisions when designing your database schema.

In essence, a primary key uniquely identifies each record in a table, while a unique key ensures uniqueness of a specific column or set of columns, but doesn't necessarily serve as the table's primary identifier. This seemingly subtle difference has significant implications for database structure and performance.

What is a Primary Key?

A primary key is a constraint that uniquely identifies each row within a database table. It's the most fundamental constraint, acting as the table's main identifier. Think of it as the social security number for a table's rows – it must be unique and cannot be NULL (empty).

Key Characteristics of a Primary Key:

• Uniqueness: Every record must have a unique primary key value. No two records can share the same primary key.
• Not Null: A primary key cannot contain NULL values. This ensures that every record in the table has a unique identifier.
• Single Column or Composite: A primary key can consist of a single column or multiple columns (composite key). The choice depends on the table's structure and the best way to uniquely identify its records.

What is a Unique Key?

A unique key, unlike a primary key, is not a mandatory constraint. It's an optional constraint that ensures uniqueness across a specific column or a set of columns within a table. This means that no two rows can have the same value in the columns assigned a unique key constraint.

Key Characteristics of a Unique Key:

• Uniqueness: Ensures uniqueness across the specified column(s).
• Allows Nulls (in most cases): Unlike a primary key, a unique key can allow NULL values, but only one row can have a NULL value in the column(s) designated as the unique key. This is database-system specific, so it's crucial to check your database system's documentation.
• Multiple Unique Keys Per Table: A table can have multiple unique keys, each enforcing uniqueness across a different set of columns.

Primary Key vs. Unique Key: A Table Summarizing the Differences

Practical Examples

Imagine a Customers table.

• Primary Key: CustomerID (an auto-incrementing integer) would serve as the primary key, uniquely identifying each customer.
• Unique Key: Email could be a unique key, ensuring that no two customers have the same email address. PhoneNumber could also be a unique key, although in reality, phone numbers might not always be unique across a customer base.

Another example: An Employees table might use EmployeeID as a primary key and have a unique key on a combination of FirstName and LastName to ensure no two employees share the same name.

Choosing Between Primary and Unique Keys

The decision of whether to use a primary key or a unique key depends on the specific requirements of your database design. Always use a primary key to uniquely identify each row. Unique keys should be used only when you need to enforce uniqueness across a column or set of columns that do not serve as the main identifier of the row. Overusing unique keys can impact performance, so use them judiciously. Careful planning and consideration are crucial for creating a well-structured and efficient database.