MySQL Primary Key vs Foreign Key

The MySQL and relational databases, both primary keys and foreign keys are important concepts used to establish relationships between tables and ensure data integrity. Let’s explore the differences between primary keys and foreign keys:

Primary Key:

1. Definition:

• A primary key is a column or a set of columns in a table that uniquely identifies each row in the table.
• It ensures that each row has a unique identifier and enforces entity integrity.

1. Uniqueness:

• Every value in the primary key column(s) must be unique across all rows in the table.
• Primary keys help prevent duplicate records in a table.

1. Nullability:

• Primary key columns cannot contain NULL values.
• They must have a value for each row.

1. Role:

• The primary key serves as the main identifier for a table.
• It’s used as a reference in foreign key relationships.

Foreign Key:

1. Definition:

• A foreign key is a column or a set of columns in one table that refers to the primary key in another table.
• It establishes a relationship between two tables, enforcing referential integrity.

1. References:

• A foreign key references the primary key of another table, creating a link between the two tables.
• It ensures that the values in the foreign key column(s) exist in the referenced primary key column(s).

1. Data Integrity:

• Foreign keys ensure referential integrity by preventing actions that would violate the relationship between tables.
• For example, a foreign key prevents inserting a row with a value in the foreign key column that does not exist in the referenced primary key.

1. Nullability:

• Foreign key columns can contain NULL values, indicating that there’s no reference to the primary key in the referenced table for that row.

Usage:

• Primary keys are used to uniquely identify records within a table, ensuring each record is distinct.
• Foreign keys are used to establish relationships between tables, ensuring consistency and data integrity when linking records from different tables.

Example:

Let’s say you have two tables: Customers and Orders. Each customer can have multiple orders. In this case:

• Customers table might have a primary key on the customer_id column.
• The Orders table might have a foreign key on the customer_id column, which references the customer_id in the Customers table.

In summary, primary keys uniquely identify records within a table, while foreign keys establish relationships between tables by referencing the primary key of another table. Together, they ensure data integrity and facilitate proper data organization in a relational database.