Understanding PRIMARY KEY and UNIQUE Constraints in SQL

When designing a relational database, maintaining the integrity and uniqueness of your data is crucial. SQL provides several constraints to help with this, among which PRIMARY KEY and UNIQUE are commonly misunderstood. Although they seem similar at first glance, both ensure uniqueness, they serve different purposes and have distinct behaviors. In this blog, we’ll explore the key differences between the PRIMARY KEY and UNIQUE constraints in SQL and when to use each.

What is a PRIMARY KEY?

A PRIMARY KEY is a column or a set of columns in a table that uniquely identifies each row. It is a fundamental component of database normalization and relational integrity.

Key Characteristics:

• Uniqueness: No two rows can have the same value in the primary key column(s).
• Not Null: A primary key cannot contain NULL values.
• One per table: Each table can have only one primary key.

Example:

CREATE TABLE employees (
    employee_id INT PRIMARY KEY,
    name VARCHAR(100),
    department VARCHAR(50)
);

Here, employee_id acts as the primary identifier for each employee.

What is a UNIQUE Constraint?

The UNIQUE constraint also ensures that all values in a column (or group of columns) are distinct. However, unlike the PRIMARY KEY, a table can have multiple UNIQUE constraints.

Key Characteristics:

• Uniqueness: Ensures all values in the column(s) are unique.
• Allows NULLs: Can include NULL values (depending on the SQL dialect, like MySQL, PostgreSQL, etc.).
• Multiple per table: You can apply more than one UNIQUE constraint in a single table.

Example:

CREATE TABLE users (
    user_id INT PRIMARY KEY,
    email VARCHAR(100) UNIQUE,
    phone_number VARCHAR(15) UNIQUE
);

Here, both email and phone_number must be unique, but neither of them serves as the main identifier (user_id does that).

Use Case Scenarios

• Use PRIMARY KEY when:
• You need to uniquely identify each record.
• The column will always have a non-null, unique value.
• You are establishing a foreign key relationship with another table.

• Use UNIQUE when:
• You want to prevent duplicate values in a column that isn’t the primary identifier.
• You have alternate keys (like username, email, or phone number).
• You want to maintain data integrity without enforcing a main row identifier.

Common Pitfalls to Avoid

• Defining multiple primary keys: This is not allowed. Use UNIQUE constraints for additional uniqueness needs.
• Assuming UNIQUE behaves exactly like PRIMARY KEY: Remember, UNIQUE allows NULL values, while PRIMARY KEY doesn’t.
• Relying only on application logic for uniqueness: Constraints are enforced at the database level, providing a stronger safeguard.

Conclusion

Both PRIMARY KEY and UNIQUE constraints are essential for ensuring data integrity in SQL databases. While they share the responsibility of enforcing uniqueness, they differ in behavior and use cases. Use a PRIMARY KEY to define the main identity of a record, and use UNIQUE to enforce additional uniqueness rules where necessary.

Understanding these differences will help you design cleaner, more robust databases that stand the test of time, and changing application logic.

To read more about How to Debug and Fix Constraint Violations in SQL, refer to our blog How to Debug and Fix Constraint Violations in SQL.