---
title: "Explain database primary key vs unique key?"  
description: "Explain database primary key vs unique key?"  
author: "Ravi Vishwakarma"  
published: 2026-01-29  
updated: 2026-02-01  
canonical: https://answers.mindstick.com/qa/116303/explain-database-primary-key-vs-unique-key  
category: "database"  
tags: ["database"]  
reading_time: 2 minutes  

---

# Explain database primary key vs unique key?

## Answers

### Answer by Anubhav Sharma

Both [**Primary Key** and **Unique Key** ensure **uniqueness**](https://answers.mindstick.com/qa/116302/what-is-database-constraints), but they are **not the same**.

### 1. Primary Key

- Uniquely identifies **each row**
- **Cannot be NULL**
- Only **one primary key** per table
- Automatically creates a **[clustered index](https://www.mindstick.com/interview/1892/what-is-a-clustered-index)** (in most DBs like [SQL Server](https://www.mindstick.com/articles/807/remove-delete-duplicate-records-or-rows-sql-server))
- Used as the **main identifier** and for **[foreign keys](https://www.mindstick.com/interview/34245/what-are-navigation-properties-and-how-do-they-relate-to-foreign-keys)**

## Example:

```plaintext
UserId INT PRIMARY KEY
```

### 2. Unique Key

- Ensures values are **unique**, but **not the main identifier**
- Allows **NULL** values (usually only one NULL, DB-dependent)
- Multiple **unique keys** allowed per table
- Creates a **non-clustered index** by default
- Not mandatory for [foreign key](https://www.mindstick.com/blog/174/foreign-key-self-reference-constraint) [reference](https://www.mindstick.com/forum/774/reference-what-does-this-error-mean-in-php) (but can be used)

## Example:

```plaintext
Email VARCHAR(100) UNIQUE
```

## Key Differences (Quick Table)

| Feature | Primary Key | Unique Key |
| --- | --- | --- |
| Uniqueness | Yes | Yes |
| NULL allowed | No | Yes (DB-dependent) |
| Count per table | Only one | Multiple |
| Index type | Clustered (default) | Non-clustered |
| Foreign key reference | Commonly used | Can be used |
| Purpose | Row identity | Data uniqueness |

## Real-world Example

```plaintext
CREATE TABLE Users (
    UserId INT PRIMARY KEY,
    Email VARCHAR(100) UNIQUE,
    AadhaarNo CHAR(12) UNIQUE
);
```

- `UserId` → **Primary Key** (main identifier)
- `Email`, `AadhaarNo` → **Unique Keys** (no duplicates allowed)

## Interview One-Liner

> **Primary Key uniquely identifies a record and cannot be NULL, while Unique Key only enforces uniqueness and can allow [NULL values](https://www.mindstick.com/forum/159818/how-to-use-the-coalesce-function-to-handle-null-values-in-sql-queries).**


---

Original Source: https://answers.mindstick.com/qa/116303/explain-database-primary-key-vs-unique-key

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