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Difference between PRIMARY KEY vs UNIQUE KEY.

Anubhav Sharma 283 27 Apr 2026

1 Answers

Anubhav Sharma 27 Apr 2026

Here’s a clear, interview-ready difference between PRIMARY KEY and UNIQUE KEY:

PRIMARY KEY

Uniquely identifies each row in a table
Cannot contain NULL values
Only one primary key per table
Automatically creates a clustered index (in most databases like SQL Server)
Used as the main identifier

Example:

CREATE TABLE Users (
    Id INT PRIMARY KEY,
    Name VARCHAR(100)
);

UNIQUE KEY

Ensures all values in a column are unique
Can contain NULL (usually only one NULL allowed, depends on DB)
Can have multiple unique keys in a table
Creates a non-clustered index by default
Used for alternate uniqueness (like email, username)

Example:

CREATE TABLE Users (
    Id INT PRIMARY KEY,
    Email VARCHAR(100) UNIQUE
);

Quick Comparison

Feature	PRIMARY KEY	UNIQUE KEY
Uniqueness	Yes	Yes
NULL Allowed	No	Yes (1 or more*)
Keys per Table	Only 1	Multiple allowed
Purpose	Main identifier	Alternate unique
Index Type	Clustered (default)	Non-clustered

* Depends on database (SQL Server allows one NULL per UNIQUE column)

When to Use

Use PRIMARY KEY → for main ID (like UserId)
Use UNIQUE KEY → for fields like Email, Phone, Username

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