--- description: What is Primary Key? A primary key constrain is a column or group of columns in a table that uniquely identifies every row in that table. The Primary key can't be a duplicate, meaning the same value c title: Difference Between Primary key and Unique key image: https://www.guru99.com/images/difference-between-primary-key-and-unique-key-1.png --- [Skip to content](#main) ## Key Difference between Primary Key and Unique Key - There can be one primary key in a table, while there can be multiple unique keys in the table. - The purpose of the primary key is to enforce entity integrity; on the other hand, the purpose of the unique key is to enforce unique data. - In primary keys, the default index is clustered, whereas in unique keys, the default index is not-clustered - The Primary key does not allow null columns, whereas the unique key allows null columns. - In the primary key, duplicate keys are not allowed, while in a unique key, if one or more key parts are null, then duplicate keys are allowed. [![Difference Between Primary key and Unique key](https://www.guru99.com/images/difference-between-primary-key-and-unique-key-1.png)](https://www.guru99.com/images/difference-between-primary-key-and-unique-key-1.png) *Difference Between Primary key and Unique key* Here, I have analyzed the difference between the primary key and the unique key and will comprehensively evaluate their pros and cons. ## What is Primary Key? A primary key constraint is a column or group of columns in a table that uniquely identifies every row in that table. The Primary key can’t be a duplicate, meaning the same value can’t appear more than once in the table. A table can have only one primary key. [Primary key](https://en.wikipedia.org/wiki/Primary_key) can be defined at the column or table level. If you create a composite primary key, it should be defined at the table level. ## Example of Creating a Primary Key The following example describes that there is a table called student. It contains five attributes: 1) StudID, 2) Roll No., 3) First Name, 4) Last Name, and 5) Email. The Roll No attribute can never contain a duplicate or null value. This is because every student enrolled in a university can have a unique roll number. You can easily identify each row of a table with the student’s roll number. So, it is considered a primary key. [![Example of Creating Primary Key]()](https://www.guru99.com/images/2/041720_1123_PrimaryKeyv2.png) *Primary Key example* ## Features of Primary Key Based on my observations, here are the important features of a primary key: - The primary key implements the entity integrity of the table. - You can keep only one primary in the table. - The primary key contains one or more table columns. - Columns are defined as not null. ## Why use Primary Key? [![Primary Key]()](https://www.guru99.com/images/2/041720_1123_PrimaryKeyv1.png) Reflecting on my practical knowledge, here are the important reasons to use a primary key: - The main aim of the primary key is to identify each and every record in the database table. - You can use a primary key when you do not allow someone to enter null values. - If you delete or update a record, the action you specified will be undertaken to ensure database data integrity. - Perform a restrict operation to reject the delete or update operation for the parent table. - Data are organized in the sequence of clustered indexes whenever you physically organize a DBMS table. **Don't Miss:** - [Entity Relationship (ER) Diagram Model with DBMS Example](https://www.guru99.com/er-diagram-tutorial-dbms.html) - [Relational Algebra in DBMS with Examples](https://www.guru99.com/relational-algebra-dbms.html) - [Difference Between File System and DBMS](https://www.guru99.com/difference-between-file-system-and-dbms.html) - [60+ DBMS Interview Questions and Answers](https://www.guru99.com/dbms-interview-questions.html) ## What is Unique Key? A unique key is a group of one or more fields or columns in a table that uniquely identify a database record. A unique key is the same as a primary key, but it can accept one null value for a table column. It also cannot contain identical values. Unique constraints are referenced by the foreign key of other tables. ## Example of Creating Unique Key Consider the same student table with the following attributes: 1) StudID, 2) Roll No., 3) First Name, 4) Last Name, and 5) Email. Stud ID can have a unique constraint where entries in the Stud ID column can be unique because each student at a university must have a unique ID number. If a student is changing universities, he or she would not have any student ID. The entry may have a null value, as only one null is allowed in the unique key constraint. [![Example of Creating Unique Key]()](https://www.guru99.com/images/2/041720_1123_PrimaryKeyv3.png) *Unique key example* ## Features of Unique key Here, I will outline the important features of a unique key: - You can define more than one unique key in the table. - By default, unique keys are in non-clustered unique indexes. - It consists of one or more table columns. - The table column can be null, but only one null per column is preferable. - A unique constraint can be easily referenced by a foreign key constraint. ## Why use Unique Key? Here are the important reasons I recommend using a unique key: - The purpose of a unique key is to make sure that the information in the column for each table record is unique. - When you allow the user to enter the null value. - [Unique key](https://en.wikipedia.org/wiki/Unique_key) is used because it creates a non-clustered index by default. - Unique key can be used when you have to keep null values in column. - When one or more than one field/columns of a table uniquely identify a record in a database table. ## Differences Between Primary Key and Unique Key In my analysis, here are the main differences between a primary key and a unique key: [![Difference between Primary Key and Unique key]()](https://www.guru99.com/images/2/041720_1123_PrimaryKeyv4.png) *Primary Key vs Unique key* | Comparison Basis | Primary Key | Unique Key | | --- | --- | --- | | Number of Key | There can be one primary key in a table | There can be multiple unique keys in the table | | NULL | It does not allow null columns. | It allows null columns. | | Index | Default index is clustered | Default Index is no-clustered | | Purpose | The purpose of the primary key is to enforce entity integrity. | The purpose of a unique key is to enforce unique data. | | Use | It is a SQL constraint that allows you to uniquely identify each record or row in the database table. | It is an SQL constraint that does not allow the same value to be assigned to two isolated records in a database table. | | Value Modification | In the primary key, duplicate keys are not allowed. | In a unique key, if one or more key parts are null, then duplicate keys are allowed. | | Syntax | Primary key can be created using syntax:
CREATE TABLE Employee
(
ID int PRIMARY KEY, 
Name varchar(255), 
City varchar(150)
)
| Unique key can be created using syntax:
CREATE TABLE Employee
(
ID int UNIQUE.
Name varchar(255) NOT NULL. City varchar(150)
)
| You can also explore: [Difference Between Primary Key and Foreign Key](https://www.guru99.com/difference-between-primary-key-and-foreign-key.html) ## How to Choose Between a Primary Key and a Unique Key - Unique keys are better when we have columns that we know shouldn’t contain duplication. - The primary key is ideal when you cannot keep null in the table. It can also be used when you have a foreign key in another table to create a relationship. #### Summarize this post with: ChatGPTPerplexityGrokGoogle AI **Stay Updated on AI** **Get Weekly AI Skills, Trends, Actionable Advice.** ##### Sign up for the newsletter Subscribe for Free You have successfully subscribed. Please check your inbox. ![AI-Newsletter]() Chosen by over **350,000+** professionals [Scroll to top](#wrapper) × Search for: ```json {"@context":"https://schema.org","@graph":[{"@type":"Organization","@id":"https://www.guru99.com/#organization","name":"Guru99","sameAs":["https://www.facebook.com/Guru99Official","https://twitter.com/guru99com"],"logo":{"@type":"ImageObject","@id":"https://www.guru99.com/#logo","url":"https://www.guru99.com/images/guru99-logo-v1-150x59.png","contentUrl":"https://www.guru99.com/images/guru99-logo-v1-150x59.png","caption":"Guru99","inLanguage":"en-US"}},{"@type":"WebSite","@id":"https://www.guru99.com/#website","url":"https://www.guru99.com","name":"Guru99","publisher":{"@id":"https://www.guru99.com/#organization"},"inLanguage":"en-US"},{"@type":"ImageObject","@id":"https://www.guru99.com/images/difference-between-primary-key-and-unique-key-1.png","url":"https://www.guru99.com/images/difference-between-primary-key-and-unique-key-1.png","width":"738","height":"250","inLanguage":"en-US"},{"@type":"BreadcrumbList","@id":"https://www.guru99.com/difference-between-primary-key-and-unique-key.html#breadcrumb","itemListElement":[{"@type":"ListItem","position":"1","item":{"@id":"https://www.guru99.com","name":"Home"}},{"@type":"ListItem","position":"2","item":{"@id":"https://www.guru99.com/dbms","name":"DBMS"}},{"@type":"ListItem","position":"3","item":{"@id":"https://www.guru99.com/difference-between-primary-key-and-unique-key.html","name":"Difference Between Primary key and Unique key"}}]},{"@type":"WebPage","@id":"https://www.guru99.com/difference-between-primary-key-and-unique-key.html#webpage","url":"https://www.guru99.com/difference-between-primary-key-and-unique-key.html","name":"Difference Between Primary key and Unique key","dateModified":"2026-10-07T19:02:31+05:30","isPartOf":{"@id":"https://www.guru99.com/#website"},"primaryImageOfPage":{"@id":"https://www.guru99.com/images/difference-between-primary-key-and-unique-key-1.png"},"inLanguage":"en-US","breadcrumb":{"@id":"https://www.guru99.com/difference-between-primary-key-and-unique-key.html#breadcrumb"}},{"@type":"Person","@id":"https://www.guru99.com/author/editorial","name":"Editorial Team","url":"https://www.guru99.com/author/editorial","image":{"@type":"ImageObject","@id":"https://www.guru99.com/images/favicon-new-logo.png","url":"https://www.guru99.com/images/favicon-new-logo.png","caption":"Editorial Team","inLanguage":"en-US"},"sameAs":["https://www.guru99.com/editorial-team"],"worksFor":{"@id":"https://www.guru99.com/#organization"}},{"@type":"NewsArticle","headline":"Difference Between Primary key and Unique key","keywords":"dbms, sql","dateModified":"2026-10-07T19:02:31+05:30","articleSection":"DBMS","author":{"@id":"https://www.guru99.com/author/editorial","name":"Editorial Team"},"publisher":{"@id":"https://www.guru99.com/#organization"},"description":"What is Primary Key? A primary key constrain is a column or group of columns in a table that uniquely identifies every row in that table. The Primary key can't be a duplicate, meaning the same value c","copyrightYear":"2026","copyrightHolder":{"@id":"https://www.guru99.com/#organization"},"name":"Difference Between Primary key and Unique key","@id":"https://www.guru99.com/difference-between-primary-key-and-unique-key.html#richSnippet","isPartOf":{"@id":"https://www.guru99.com/difference-between-primary-key-and-unique-key.html#webpage"},"image":{"@id":"https://www.guru99.com/images/difference-between-primary-key-and-unique-key-1.png"},"inLanguage":"en-US","mainEntityOfPage":{"@id":"https://www.guru99.com/difference-between-primary-key-and-unique-key.html#webpage"}}]} ```