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6 Types of Keys in DBMS

In the tangible realm, an extensive quantity of data is accessible. Presently, the DBMS requires numerous tables to accommodate this data. These tables may contain many duplicate records, both sorted and unsorted. Consequently, retrieving a distinct or particular record from these tables without constraints or limitations poses a formidable challenge.


A novel concept called “Keys” has emerged to address these challenges. Keys serve the purpose of guaranteeing the absence of duplicate records within rows or tuples. Let’s delve into understanding of keys in DBMS. 


Table of Content:

Explanation of Keys in DBMS

In a DBMS, KEYS are attributes or sets of attributes that enable the identification of a row or tuple within a relation or table. They establish connections between different tables and assist in uniquely identifying a row by utilizing one or more columns in the table. Keys play a vital role in locating distinct records or rows within a table, and they serve as a means to find a unique record or row within the database. In this article let’s explore the importance of keys in DBMS and its major types.

Importance of Keys in Maintaining Database Integrity

Keys are utilized in a database to establish different types of integrity constraints. Conversely, a table is a compilation of records pertaining to diverse events within a given relation. Many records may exist within these tables, some of which could be duplicates.


Hence, it becomes necessary to uniquely and distinctly identify each of these records, eliminating any duplications. This predicament is effectively resolved through the implementation of keys. Keys in DBMS play a pivotal role in ensuring the uniqueness and separation of records, alleviating the associated challenges.

Types of Keys in DBMS

Now we have understood what is a key in DBMS, let’s look at the major types of keys in DBMS in detail.


1. Primary Key 

This type of keys in DBMS refers to a column that uniquely identifies all the records within that table. A table has one primary key only, and this key must not contain repeated or duplicated values across its rows. Each value within the primary key must be unique, with no repetitions allowed.

Applying the PK (PRIMARY KEY) constraint to a column or set of columns ensures that they cannot have null values or duplicates. Furthermore, any foreign key that references the primary key is unable to modify the values present in the primary key.

Properties of a Primary Key

  • The Primary Key field shouldn’t be left NULL; the Primary Key column must contain a value. 
  • In that column, no two rows in the table may contain identical values. 
  • If a foreign key in a DBMS refers to the primary Key, no value may be altered or modified in this primary key column.

Importance of Primary Key In DBMS

For the following reasons, the primary key is crucial in a database management system (DBMS): 


  • It guarantees that each entry in a table can be uniquely identified.
  • The primary key supports data integrity by prohibiting redundant or duplicate records by ensuring uniqueness.
  • Primary keys are often indexed by the DBMS, enhancing the speed of data retrieval operations. 
  • The primary key has a critical role to play in building associations between tables.

Examples of Primary Keys 

In this example, the student ID is the primary key.



Roll No First Name Last Name Email


11 Tom Prince abc@gmail.com
2 12 Nick Wright


3 13 Dana Natan


Guidelines for Choosing a Primary Key

  1. Uniqueness is the most crucial element while selecting this key in DBMS. It means that this column’s value does not occur in any other table row.
  2. The definition and values shouldn’t be altered. While altering a PK column value would need updating all referenced rows in the child (related) table, altering a PK’s columns would necessitate redefining all pertinent foreign keys.
  3. If a composite primary key is used, no single column or smaller group of columns should be able to identify each individual uniquely.
  4. Use as few columns as you can, and if you can, pick columns whose values are simple to read and recall.

2. Candidate Key

Candidate keys play a vital role in upholding the integrity and consistency of a database. The purpose of this key in DBMS is to guarantee each row’s uniqueness and independent identification within a table. Additionally, candidate keys enforce relationships between tables, ensuring data integrity and maintaining overall database consistency.


Properties of a candidate key

  • It must have distinct values.
  • A candidate key in SQL can have a variety of qualities, but it must not include null values and must have at least the bare minimum of fields to guarantee uniqueness.
  • Create a unique identifier for each table record.

Examples of candidate keys


Roll No First Name Last Name Email
1 11 Tom Prince



12 Nick Wright xyz@gmail.com
3 13 Dana Natan



The candidate keys Roll No., Stud ID, and Email in the table enable us to identify each student record uniquely.

Differences Between Primary and Candidate keys

Primary Key Candidate Key
A primary key is a column that allows each entry in a database to be identified only once. It is selected as the primary key for the table from the list of potential keys. A column that can uniquely identify each record in a database is sometimes referred to as a candidate key. It might be used as the primary key.
A primary key is a unique identifier for a record in a database table. A candidate key is also a unique identifier for a record in a database table.
There can be only one primary key in a table. There can be multiple candidate keys in a table.
The primary key enforces entity integrity, Candidate keys are potential candidates
The primary key is chosen by the database designer or administrator. A candidate key can be chosen from the set of candidate keys for a table.

Importance of candidate keys in DBMS

These keys in DBMS are essential for preserving a database’s consistency and integrity. They are used to enforce connections between tables and guarantee that each row in a database is distinct and can be recognized independently.


3. Super Key

The collection of all keys enabling us to recognize every row in the table is a super key. This type of key in DBMS specifies that all the table columns that may identify the columns uniquely function as the super keys.


Properties of a super key

  • A super key must ensure that each record in a table is unique.
  • Nevertheless, the minimal collection of characteristics that can guarantee uniqueness should be a super key.
  • Multiple subsets that are likewise regarded as super keys can exist for a super key.

Importance of super keys in DBMS

A super key’s only function is to identify specific table rows. Since a column with duplicates won’t be able to identify a unique row, you may often not tell which table it belongs to by picking any random column. Data retrieval is simplified, and this uncertainty is eliminated with this key in DBMS.


Also read about : Best Data Visualization Tools to Know in 2024


4. Foreign Key

To create connections between two accessible tables, we employ a foreign key. Every value in a column or collection must match the primary key in the referential table for the foreign key to function. We can preserve referential integrity and data integrity with the aid of a foreign key.


Properties of a foreign key

  • It is a key that serves as both a secondary key and a primary key in two different tables.
  • At any given time, it combines two or more relations.
  • They serve as cross-references for the tables.

Examples of foreign keys

Dept code

Dept name







Teacher ID

Fname Lname




B017 Sara






In this Key in DBMS example, we have two tables: instructor and department at a school. However, it is hard to distinguish which instructor is assigned to which division. 


We can link the two tables in this table by adding the Foreign Key in Deptcode to the Teacher name. 


Importance of foreign keys in DBMS


Data in one table is connected to another through foreign keys. To develop a manner of cross-referencing two columns, a foreign key column in one table links to the column in another table with unique values.


5.Alternate Key

A key in DBMS might be selected as the main Key in a table in multiple ways. Any key that has the potential to replace the primary key but is not yet the primary key is considered an alternate key. It’s a prospective main key that hasn’t been selected yet.


Also read about – Data Mining and Data Warehousing.


Properties of an alternate key

  • Alternate keys refer to all keys that are not main keys.
  • It’s a backup key.
  • It has two or more fields that allow it to recognize two or more records.
  • These criteria are reiterated.

Examples of alternate keys



Roll No First Name Last Name Email
1 11 Tom Prince



12 Nick Wright xyz@gmail.com
3 13 Dana Natan



StudID, Roll No., and Email serves as the main keys. However, because StudID is the main key, Roll No. and Email is now the secondary key.


Importance of alternate keys in DBMS


Since an alternate key is identical to a candidate key, its purpose and function are the same. This indicates that the columns in a table that may be used to identify each record in the database separately are additionally identified using an alternative key.


6.Compound Key

This key in DBMS contains two or more characteristics that recognise a specific record exclusively. It’s conceivable that none of the columns in the database are unique on their own. However, when paired with the additional column or columns, the composite key combination becomes unique. Each record in the table is to be uniquely identified using the database’s compound key.


Properties of a compound key

  • Each record in a table must be guaranteed unique by a compound key.
  • The bare minimal collection of characteristics necessary to assure uniqueness should be a compound key.
  • A compound key attribute order is essential.

Examples of compound keys

Order No

Product ID Productname Quantity


JAP1024 Mouse


B005 JHD1345 USB



KDL0973 LCD Monitor 20
B002 FRG1204 USB



JUW1287 Laser printer



Below is the example of this key in DBMS:


OrderNo and ProductID cannot be the main key since they do not uniquely identify a record. A compound key combining the Order ID and Product ID might be utilized to identify each record uniquely.


Importance of compound keys in DBMS

Compound keys are always constructed from two or more other tables’ primary keys. Both keys uniquely identify data in their respective tables, but both are required to do so in the table utilizing the compound key.

Comparison of 6 Types of keys in DBMS

Here’s a comparison between different types of keys in a relational database management system (DBMS) presented in a table format:


Key Type Definition Uniqueness Null Values Purpose Index Usage Alteration
Primary Key Uniquely identifies each record in a table. Required Not Allowed Data integrity, record identification. Creates Unique Index Within same table May be complex, impact other tables.
Foreign Key Establishes a relationship between tables. Not Required Allowed Data consistency, relationship maintenance. May create Index Between tables More flexible for maintenance.
Candidate Key Alternative unique keys that could be primary keys. Required Not Allowed Backup primary key options. May create Index Within same table May become primary key.
Super Key Combination of attributes that uniquely identify. Required Allowed Conceptual, not enforced. May create Index Conceptual use N/A
Composite Key Combined attributes used as a single key. Required Not Allowed Specialized unique identification. Creates Composite Index Within same table May be complex, impact performance.
Unique Key Ensures column(s) have unique values. Required Allowed Enforce uniqueness, no primary key. Creates Unique Index Within same table May be used as primary key.
Surrogate Key Artificial keys assigned for record identification. Required Not Allowed Enhanced data privacy, data warehousing. Creates Unique Index Within same table Generally static, non-changing.

In this guide we have covered all about the keys in DBMS, their major types with example. When developing a database, keys and functional dependencies are crucial components. The distinction between excellent and poor database design may also be made using these ideas. Normalization is the final step in eliminating redundancies and improving database performance, which uses all the concepts discussed in this article. Also check out this Business Analytics course by Hero Vired to get master in niche.


About the Author:


Hero Vired collaborates with industry professionals from respected companies and colleges to inspire and push students by providing them with Full Stack Development Course and real-world, industry-relevant experience.


We employ a key to create different kinds of integrity restrictions in a database. On the other hand, the records of different occurrences for any relation are compiled in a table. These documents might number in the thousands; some might even be identical.
A row in a table (or relation) can be uniquely identified by one or more properties, known as a key. When creating connections between the various columns and tables in a relational database, we employ a key. "key values" refers to the specific values that make up a key.
Keys are of seven broad types in DBMS:
  • Candidate Key
  • Foreign Key
  • Primary Key
  • Alternate Key
  • Super Key
  • Compound Key

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