Primary Key vs. Foreign Key: 9 Important Differences

Primary Key vs. Foreign Key  

Keys are essential in a relational database to keep tables connected or uniquely pinpoint table data. A primary key uniquely identifies data, so no two rows share the same primary key and cannot be null. Whereas a foreign key links two tables together.  

A primary key from one table serving as a foreign key in another is a common way to enforce data integrity. It ensures that the data in the referencing table (the one with the foreign key) has a valid link to the referenced table (the one with the primary key). This prevents orphaned records and maintains consistency across the database.  

Primary Key  

A primary key identifies each row in a table. It is contained in a parent table. A primary key can be an individual column or a group of columns. To insert, update, restore, or delete operations in a database table, the table must have a primary key.  

Having a primary key is significant for:  

• Uniquely identifying rows in a table or records to easily retrieve, update, or delete.  
• The primary key in DBMSs like MySQL and Oracle is usually an auto-incrementing integer. This means the database automatically gives each new record a new number, making sure every row has its unique identifier. 

Foreign Key  

A foreign key is a reference point in a relational database that establishes relationships between two tables, ensuring data consistency and integrity. Unlike primary key, it is present in a child table.

When you apply a foreign key constraint to a table’s column, it must reference the primary key of another table’s column. This linkage maintains a relational structure, connecting data across different tables. You can specify these relationships using the “references” keyword to signal to the database that a specific column (the foreign key) must match an existing value in the primary key of another table.  Doing so enforces referential integrity and guarantees that data references are valid from one table to another.

Foreign keys fulfill multiple needs in a database model:

• Foreign keys ensure data integrity by maintaining consistency, completeness, and accuracy across related tables.  

• They optimize query performance by facilitating efficient query plans, accelerating data retrieval, and enhancing the relationships between tables.  

• Foreign keys are essential in establishing relationships between tables, enabling linked data storage and retrieval across multiple tables. 

Comparing Primary Keys and Foreign Keys 

Both primary and foreign keys play important but distinct roles in maintaining data integrity and establishing meaningful connections in databases. While both involve identifying data points, they serve different purposes and possess unique characteristics. Here’s how primary and foreign keys compare across several key factors:

1. Purpose:

The primary key’s sole purpose is uniquely identifying each table record. In contrast, the foreign key references the primary key of another table, establishing a relationship and enabling data retrieval across different tables. This allows you to connect related information and see the unified overview within your database.. 

2. Uniqueness:

The primary key must hold a unique value for every record in the table. There can be no duplicates – each record needs its distinct identifier.

Uniqueness within its table is optional for a foreign key. But it must reference a unique value in the primary key of the table it points to. It can only connect to a single, well-defined point on the other side. 

3. Nullability:

Null values are generally not allowed in the primary key. Every record needs a defined primary key value to ensure no missing identifiers and prevent confusion when referencing specific data points.

Depending on the relationship between the tables, a foreign key allows null values. For example, a customer order can have a foreign key referencing a “shipping address,” but the address field will be null if the order hasn’t been shipped yet. 

4. Data Integrity Enforcement:

By its very nature, the primary key enforces data integrity within its table. Uniqueness ensures no duplicate records exist, and the absence of null values prevents missing identifiers. 

Foreign keys are vital in maintaining data integrity across tables. The reference to a valid primary key in another table helps prevent orphaned records (records with foreign fundamental values that don’t correspond to any existing data in the referenced table). This creates consistency and prevents broken relationships within your database.

5. Updatability and Deletability:

Due to its role as the unique identifier, the primary key is typically designed to be updated sparingly. Changing the primary key value could disrupt relationships with other tables.

Users can update foreign key values if the new value remains a valid primary key in the referenced table. However, deleting a record in the referenced table can impact other tables’ corresponding foreign key values, depending on the chosen referential integrity constraints. 

Primary Key vs. Foreign Key with an Example 

Let’s consider an online store as an example to better comprehend the difference between primary and foreign keys. You need a primary key to uniquely identify each product, like a ‘Product ID’ that will ensure no two products have the same code.  

For- example, the ‘Products table’ has details about items, such as a unique Product ID (e.g., D972), the product’s Name (e.g., Jeans), and its Description (e.g., Cotton Jeans). It includes a Category ID, which links to the Categories table.  

The Categories table categorizes products. It uses a Category ID as its primary key (e.g., Q204101) and has a Name field, for example, “Clothing”. The foreign key in the Products table (Category ID) references the Category ID (primary key) in the Categories table. This connection ensures data consistency.  

If the category name changes from “Clothing” to “Apparel” in the Categories table, it will automatically update in the Products table for all related products (Jeans in this case).  

Primary Key vs. Foreign Key: 9 Important Differences

Primary Key	Foreign Key
A column or a set of columns that identifies each row in a table.	A column or a multiple columns in one table that refers to the primary key in another table.
Must contain unique values; no duplicates are allowed.	Can contain duplicate values; typically refers to primary key values in another table.
There is only one primary key per table.	Multiple foreign keys can exist in a table, depending on relationships.
Ensures data integrity and entity integrity (each row is uniquely identifiable).	Establishes and maintains referential integrity between related tables.
They are automatically indexed by default (in most DBMSs).	It may or may not be indexed automatically; the index is recommended for performance.
Typically, a numeric or unique identifier.	Matches the primary key data type it references.
The primary key constraint ensures uniqueness and is not null.	Foreign key constraint ensures referential integrity (values must exist in the referenced table).
They are used to identify rows when joining tables uniquely.	They are used to establish relationships and enforce constraints during joins.
Changes are restricted if the primary key is referenced as a foreign key elsewhere (depending on cascade options).	Values can be updated or deleted, typically with cascade options to maintain referential integrity.

Types Of Keys in a Relational Database Model (DBMS) 

Speaking of primary and foreign keys, there are several more types of keys in a database management system. Implementing these keys correctly in SQL for the appropriate database helps eliminate redundancy and helps with data analysis. Proper identification of these keys enhances database accuracy, improving results. Let’s quickly go through these: 

1. Primary Key  

A primary key in a DBMS is a single column or a combination of columns in a table that uniquely identifies each record in that table. A table can have only one primary key, which must have unique values with no repetitions across all rows.  

2. Super Key  

A super key is one key or a group of keys that can uniquely identify each row in a table.. It means any combination of columns that uniquely determines all other columns in the table qualifies as a super key. The super key includes all possible keys that can uniquely identify rows. A primary key is chosen from these super keys to identify each row in the table uniquely. 

3. Candidate Key  

Candidate keys uniquely identify table rows, acting much like primary keys with all the same properties. A table selects its primary key from among its candidate keys. While there can be multiple candidate keys, none can be empty, ensuring each carries unique information and value. A group of attributes can also collectively function as candidate keys.

4. Alternate Key  

A table may have multiple primary key candidates but selects only one. The keys not chosen as the primary key are known as alternate keys.

5. Foreign Key 

Foreign keys link two tables by requiring each value in one column or column to match the primary key in another referential table. They ensure relationships between related but not identical information.

6. Composite Key  

A composite key combines two or more attributes to identify each row in a table uniquely. Although these attributes might not be unique, their combination guarantees uniqueness. This key is also known as a concatenated key.

7. Unique Key  

A unique key, consisting of one or more columns, uniquely identifies each row in a table, requiring all values in the key to be unique. Unlike a primary key, a unique key can include one null value, while a primary key allows no null values.

In DBMS, in addition to the seven standard key types, there’s also a type called Artificial Keys. An artificial key, or a surrogate key, has no business relevance or meaning. Still, it handles data management issues, such as when no attribute fully meets primary critical criteria or when primary keys become overly complex.

Conclusion  

Understanding the roles of primary keys and foreign keys is essential for maintaining a well-organized and efficient relational database. Effective implementation of these keys allows the database to perform with increased efficiency, accuracy, and consistency. They also improve data management and application development processes.  

Astera is a unified data management platform that simplifies database design by establishing relationships between entities using the correct primary and foreign keys. It ensures tables are joined correctly, and entity-relationship types like many-to-many or parent-child are accurately defined.  

Astera offers intuitive query editor commands for managing foreign and primary keys. It automatically identifies these keys and allows users to define them manually using a user-friendly layout builder, facilitating efficient database management and enhancing overall data integrity and usability. 

To learn more about how Astera streamlines the handling of primary and foreign keys, contact us or schedule a demo today.