Difference Between Primary Key and Foreign Key

Introduction

Keys are incredibly important in the world of database management systems (DBMS) for preserving data integrity and establishing connections between tables. The main key and the foreign key are the two basic types of keys. These keys have different functions and are crucial for creating reliable and effective databases. This article aims to clarify the distinction between primary and foreign keys, as well as their applications and importance in DBMS.

Overview

A field or set of attributes that uniquely identifies a record within a table in relational databases is known as a key. It guarantees data integrity and enables speedy data retrieval. Keys establish relationships between tables, facilitating data consistency and integrity. Two key types widely used are the primary key and the foreign key. Further, you’ll learn more about the difference between primary key and foreign key and unique key.

What are Keys in SQL?

In SQL, keys are used to uniquely identify records within a table. They ensure data integrity and enable efficient data retrieval. Depending on the need, keys can be composite or single-field. To further comprehend this, let's look at an illustration.

Let's say we have the following fields in a table called "Employees":

EmployeeID (Primary Key)

FirstName

LastName

DepartmentID (Foreign Key)

Here, the EmployeeID field serves as the primary key, uniquely identifying each employee. The DepartmentID field acts as a foreign key, establishing a relationship between the "Employees" table and the "Departments" table. The primary and foreign keys work together to maintain data integrity and consistency.

What is a Primary Key?

A primary key is a field or combination of fields in a table that uniquely identifies each record. It ensures that there are no duplicate records and guarantees data integrity. Typically, a primary key is chosen from the available candidate keys in a table. Here's the difference between primary key and foreign key example:

Consider a table called "Students" with the following fields:

StudentID (Primary Key)

FirstName

LastName

Age

The StudentID field, in this instance, acts as the primary key, guaranteeing that every student has a distinct identification. The StudentID is unique to each student, preventing data duplication and facilitating quick access to student-specific data.

What is the Use of Primary Key?

The primary key serves multiple purposes in a database:

• Uniquely identifies records: The primary key ensures that each record in a table has a unique identifier. This uniqueness helps in distinguishing individual records from one another.
• Ensures data integrity: By preventing duplicate records, the primary key ensures data integrity within a table. It guarantees that no two records have identical primary key values.
• Facilitates data retrieval: The primary key allows for the efficient retrieval of specific records. It serves as a reference point for searching and locating data.

To illustrate the use of a primary key, let's consider the "Students" table mentioned earlier. Suppose we want to retrieve information about a specific student with StudentID 1234. The primary key enables us to quickly locate and retrieve the relevant record.

What is a Foreign Key?

A foreign key is a field or combination of fields in a table that establishes a link or relationship between tables. It refers to the primary key of another table, creating a connection between them. Foreign keys enable the implementation of referential integrity and maintain data consistency across related tables. Let's delve into an example:

Continuing with the previous example, we have a table called "Departments" with the following fields:

DepartmentID (Primary Key)

DepartmentName

Let's now use a foreign key to link the "Employees" and "Departments" tables together. The DepartmentID field in the "Employees" database functions as a foreign key that refers to the primary key in the "Departments" table.

What is the Use of Foreign Key?

The foreign key serves several purposes in a database:

• Establishes relationships between tables: The foreign key establishes a link between tables, enabling the creation of relationships. It connects data across different tables, facilitating data consistency and integrity.
• Enforces referential integrity: By referencing the primary key of another table, the foreign key ensures that the linked values exist. It prevents orphaned records by enforcing referential integrity.
• Supports data integrity: The foreign key helps maintain data integrity by ensuring that data dependencies are preserved. It prevents inconsistent or invalid data from being inserted into related tables.

To illustrate the use of a foreign key, consider the "Employees" and "Departments" tables. The DepartmentID field in the "Employees" table acts as a foreign key, referencing the primary key (DepartmentID) of the "Departments" table. This connection allows us to establish relationships between employees and their respective departments.

Difference between Primary Key and Foreign Key

Although both primary keys and foreign keys are key components in database design, they serve different purposes and exhibit distinct characteristics. Here is the key difference between primary key and foreign key definition:

Definition and Purpose:

• Primary Key: A primary key is a unique identifier for each record within a table. It ensures data integrity and enables efficient data retrieval.
• Foreign Key: A foreign key establishes a relationship between two tables by referencing the primary key of another table. It maintains referential integrity and supports data consistency across tables.

Uniqueness:

• Primary Key: The primary key must be unique within a table. It ensures that each record has a unique identifier, preventing data duplication.
• Foreign Key: The foreign key references the primary key of another table, and it may contain duplicate values within the table. It represents the link between tables rather than guaranteeing uniqueness within the table.

Data Modification:

• Primary Key: The primary key values are typically immutable, as they serve as permanent identifiers for records. Changing the primary key value is discouraged due to potential data integrity issues.
• Foreign Key: The foreign key values can be modified, allowing for changes in the relationships between tables. Updating the foreign key value in one table can modify the associated record in the referenced table.

Table Association:

• Primary Key: The primary key is associated with the table where it is defined. It uniquely identifies records within that table.
• Foreign Key: The foreign key is associated with a different table than where it is defined. It establishes a relationship with the referenced table, connecting data across tables.

Dependency:

• Primary Key: The primary key is independent and does not rely on other tables or their keys. It uniquely identifies records within a table.
• Foreign Key: The foreign key relies on the referenced table and its primary key. It establishes a dependency on the primary key values of another table.

Constraints:

• Primary Key: The primary key enforces the uniqueness and non-nullability of its values within a table. It is often defined as the table's primary index.
• Foreign Key: The foreign key ensures that the referenced values exist in the referenced table's primary key. It may also specify cascading actions for data modification or deletion.

Primary Key vs Foreign Key Comparison Chart

To summarize the differences between primary keys and foreign keys, refer to the following chart:

Difference between Primary Key and Unique Key

The primary key and unique key are both used to enforce uniqueness in a table. However, there are some notable differences between them:

Number of Keys:

• Primary Key: A table can have only one primary key. It serves as the main identifier for the records within the table.
• Unique Key: A table can have multiple special keys. Each unique key enforces uniqueness on its respective column(s), allowing for different combinations of unique values.

Null Values:

• Primary Key: A primary key cannot contain null values. Every record in the table must have a primary key value.
• Unique Key: A unique key can contain null values. However, if a column with a unique key constraint allows null values, it can have multiple records with null values.

Purpose and Usage:

• Primary Key: The primary key uniquely identifies each record in the table. It ensures data integrity, facilitates data retrieval, and serves as a reference point for relationships.
• Unique Key: The unique key enforces uniqueness on a column or combination of columns. It can be used to maintain data integrity and enforce business rules but does not have the same relationship significance as the primary key.

Difference between Foreign Key and Reference Key

The terms "foreign key" and "reference key" are often used interchangeably. However, there is a subtle difference between them:

Definition:

• Foreign Key: A foreign key is a field or combination of fields in a table that references the primary key of another table. It establishes a relationship between the two tables.
• Reference Key: A reference key is a field or combination of fields in a table that refers to another table's primary key or unique key. It serves as a reference point for connecting tables.

Relationship:

• Foreign Key: The foreign key establishes a relationship between two tables. It connects data across tables and helps maintain referential integrity.
• Reference Key: The reference key does not establish a relationship explicitly. It acts as a reference point for connecting tables but does not enforce referential integrity.

Difference between Primary Key and Foreign Key in DBMS

In the context of DBMS, the primary key and foreign key have the following distinctions:

Purpose:

• Primary Key: The primary key uniquely identifies each record within a table. It ensures data integrity and facilitates efficient data retrieval.
• Foreign Key: The foreign key establishes a relationship between two tables by referencing the primary key of another table. It maintains referential integrity and supports data consistency across tables.

Constraints:

• Primary Key: The primary key enforces uniqueness and non-nullability within a table. It often serves as the table's primary index.
• Foreign Key: The foreign key ensures the existence of referenced values in the referenced table's primary key. It may also specify cascading actions for data modification or deletion.

Difference between Primary Key and Foreign Key in SQL

In SQL, the primary key and foreign key have the following disparities:

Definition:

Primary Key: In SQL, the primary key is a field or combination of fields that uniquely identify records within a table. It ensures data integrity and enables efficient data retrieval.

Foreign Key: In SQL, the foreign key is a field or combination of fields that reference the primary key of another table. It establishes relationships between tables and supports referential integrity.

Syntax:

Primary Key: In SQL, the primary key is defined using the "PRIMARY KEY" constraint after the field declaration. For example, "PRIMARY KEY (EmployeeID)."

Foreign Key: Following the field declaration in SQL, the "FOREIGN KEY" constraint is used to specify the foreign key, which is then followed by the "REFERENCES" keyword, the name of the referenced table, and its primary key. As an example, use "FOREIGN KEY (DepartmentID) REFERENCES Departments (DepartmentID)."

Conclusion

In conclusion, primary keys and foreign keys play crucial roles in database design and management. The primary key uniquely identifies records within a table, ensuring data integrity and facilitating efficient data retrieval. On the other hand, the foreign key establishes relationships between tables by referencing the primary key of another table, maintaining referential integrity and data consistency. Understanding the differences between primary keys and foreign keys is essential for designing robust and efficient databases that maintain data integrity and support complex relationships.

FAQs

1. Can a foreign key be a primary key?

Yes, a foreign key can also serve as a primary key in certain cases. This occurs when a table has a self-referencing relationship, where a field references its primary key.

2. Can a primary key and a unique key be in the same column?

Yes, a column can have both a primary key constraint and a unique key constraint. However, in most cases, it is sufficient to have either a primary key or a unique key on a column, as they both enforce uniqueness.

3. Can a primary key and a foreign key have different data types?

Yes, a primary key and a foreign key can have different data types. The data type of a primary key is typically chosen based on the requirements of the table, while the data type of a foreign key is determined by the referenced primary key.

4. Can a primary key and a foreign key have the same name?

Yes, a primary key and a foreign key can have the same name within their respective tables. However, it is common practice to give them distinct and descriptive names to enhance clarity and maintainability in the database structure.