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Difference Between Primary Key and Foreign Key

In the world of relational databases, understanding the difference between primary key and foreign key is essential. This is because primary keys and foreign keys are two very important components of any database, as they help define the structure of a database as well as maintain data integrity.

 

The key differences between the primary key and foreign key is that, the primary key is about uniquely identifying records within a table, while the foreign key establishes relationships between tables and enables the linking of data across multiple tables. Both keys play crucial roles in maintaining data integrity, consistency, and relational structure in a database.

 

In the section below, we’ll discover more differences between the primary key vs foreign key in detail.

 

So, let’s get started.

 

What is the Primary Key?

Before knowing the difference between primary key vs foreign key, let’s understand what they mean. A primary key is a special attribute or field within a database table that uniquely identifies each record or row in that table. It acts as a distinctive label for every information stored in the table, ensuring that no two records have the same identifier. This unique identification helps locate, access, and manipulate specific data within the table.

 

Primary keys are used for a variety of purposes, including maintaining data integrity and integrity constraints, ensuring efficient record lookup, forming relationships between different tables in a database, and providing an effective way to ensure the uniqueness of records. The business analytics course will teach you just how important having a primary key in your database is.

 

Examples of Primary key

Let’s understand the difference between primary key and foreign key with the help of examples:

 

Employee ID First Name Last Name Department
001 John Smith Sales
002 Emily Johnson HR
003 Michael Williams Marketing
004 Sarah Davis Operations

 

In this example, the “Employee ID” column is the primary key for the “Employees” table. Each employee has a unique ID assigned to them, ensuring that no two employees share the same ID. The primary key column uniquely identifies each record within the table, allowing for easy access and manipulation of specific employee data. 

 

Most of the primary keys in DBMS are numerical, but they can also be text data or a combination of both. Read on & learn about the difference between primary key and foreign key.

What is Foreign Key?

Before knowing the difference between primary key vs foreign key, let’s understand what they mean. A foreign key is a relational database theory that connects and establishes relationships among different tables by referring to the primary key of another table. This linking enables sharing and exchanging data between the two tables. The purpose of using foreign keys is to enforce referential integrity and maintain data consistency across tables. It ensures that the values stored in the foreign key field of one table are also present in the primary key field of another table.

 

A foreign key is usually composed of multiple columns used to identify a row in the related table uniquely. The number of columns used depends on how many columns make up the primary key in the related table. 

 

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Examples of Foreign key

Let’s understand the difference between primary key and foreign key with the help of examples:

 

Here’s an example of two tables, “Students” and “Courses,” with a foreign key relationship explained. Consider a table named “Students” that stores information about students and another table named “Courses” that stores information about different courses offered. Let’s establish a relationship between these two tables using foreign keys. Below, we’ll look at the difference between primary key and foreign key.

 

Table: Students

Student ID First Name Last Name Age Grade
001 John Smith 18 A
002 Emily Johnson 19 B
003 Michael Williams 20 A

 

Table: Courses

Course ID Course Name Instructor
101 Mathematics Prof. Anderson
102 English Prof. Thompson
103 Science Prof. Parker

 

To establish a relationship between these tables, we can introduce a foreign key in the “Students” table that references the primary key of the “Courses” table. Let’s add a column called “Course ID” as a foreign key in the “Students” table.

 

Table: Students

Student ID First Name Last Name Age Grade Course ID
001 John Smith 18 A 101
002 Emily Johnson 19 B 102
003 Michael Williams 20 A 103

Major Difference between Primary Key and Foreign Key

Let’s deep dive and understand the major difference between Primary Key and Foreign Key in detail:

 

Basis Primary Key Foreign Key
Definition A primary key is a unique identifier for each record in a table. A foreign key establishes a relationship between tables by referencing the primary key of another table.
Basic Ensures uniqueness and data integrity within a single table. Establishes relationships and maintains referential integrity between tables.
NULL Does not allow NULL values in the primary key field. Allows NULL values in the foreign key field, indicating a missing or unspecified relationship.
Count Only one primary key can exist per table. Multiple foreign keys can exist within a table, depending on the relationships with other tables.
Duplication No duplicate primary key values are allowed within the table. Can contain duplicate foreign key values, reflecting multiple records associated with the same reference.
Indexing Primary keys are automatically indexed to enhance data retrieval speed. Foreign keys can be indexed but are not automatically indexed.
Deletion Deleting a record with a primary key constraint can affect referential integrity in related tables. Deleting a record with a foreign key constraint can be restricted or cascade to maintain integrity.
Insertion Each new record must have a unique primary key value assigned. The foreign key can reference an existing primary key value or be NULL if the relationship is optional.
Temporary table Primary keys can be applied to temporary tables. Foreign keys can also be applied to temporary tables to establish relationships.
Relationship Primary keys define the basis for establishing relationships with other tables. Foreign keys establish relationships and connect data between related tables.

 

This table summarizes the major difference between primary and foreign keys, highlighting their definitions, basic functionalities, NULL values treatment, duplicates handling, indexing, impact on deletion and insertion operations, applicability to temporary tables, and their roles in establishing relationships between tables.

 

Here are some additional differences between primary key and foreign key, presented in bullet point form:

 

Primary Key

  • A primary key is defined within a single table and uniquely identifies each record in that table.
  • Its purpose is to ensure data integrity, enforce uniqueness, and provide a reliable means of identifying individual records.
  • The primary key value cannot be null or duplicated within the table.
  • Primary keys are often used to establish relationships with other tables through foreign keys.

Foreign Key

  • A foreign key is a way to create connections between tables by using the primary key of one table as a reference. It represents a dependency or association between two tables and maintains referential integrity.
  • Foreign keys can accept null values, allowing for optional or unspecified relationships.
  • They can be duplicated within a table, reflecting multiple records associated with the same reference.

Primary Key Vs. Foreign Key: Uses

Here are some differences between primary key and foreign key in terms of uses:

 

Primary Key Uses

  • The primary key identifies each record within a table uniquely.
  • It ensures data integrity by enforcing uniqueness and non-null values, which is crucial in the application of data structures.
  • Primary keys are essential for maintaining the integrity and reliability of data in a table.
  • They provide a means of precise data retrieval and manipulation within a single table.
  • Primary keys serve as the basis for establishing relationships with other tables through foreign keys.

Foreign Key Uses

  • The foreign key is used to establish relationships between tables.
  • It refers to a connection or relationship between two tables that ensures data consistency and accuracy.
  • Foreign keys maintain referential integrity by ensuring that the values in the foreign key field correspond to valid primary key values in the referenced table.

The Bottom Line

In this guide we have understood the major differences and similarities between primary vs foreign keys. The primary and foreign keys are important concepts and tools used in relational database management systems (RDBMS). The primary key identifies each record in a table, while the foreign key is used to establish relationships between tables. Both keys are essential components of any database, as they provide structure and integrity to the stored data. 

 

Learning the differences between these two types of keys can help you more effectively manage your databases and ensure that your data remains accurate and secure.

FAQ's

Primary keys uniquely identify records within a table, while foreign keys establish relationships between tables.
he decision to use a primary or foreign key depends on the specific requirement and purpose within a database.
  • Primary keys are used within a single table to identify records and ensure data integrity uniquely.
  • Foreign keys establish relationships between tables, connect data, and maintain referential integrity.
While primary keys and foreign keys have distinct roles, they also share some similarities:
  • Both primary keys and foreign keys play crucial roles in establishing relationships between tables in a relational database.
  • They contribute to maintaining data integrity by ensuring the accuracy and consistency of data across related tables.
There is difference between primary key and foreign key with examples: Examples of primary keys:
  • Student ID in a table of students
  • Order Number in a table of orders
  • Employee ID in a table of employees
Examples of foreign keys:
  • Customer ID in a table of orders, referencing the primary key of the customer table
  • Department ID in a table of employees, referencing the primary key of the department table
  • Book ID in a table of sales, referencing the primary key of the book table

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