A primary key and a foreign key are both important concepts in relational databases, but they serve different purposes:

Primary Key:
A primary key uniquely identifies each record (row) in a table.

It must be unique and cannot be NULL.

Each table can have only one primary key, which may consist of one or more columns.

Example:
In a Students table, student_id can be the primary key because it uniquely identifies each student.

Foreign Key:
A foreign key is a field (or collection of fields) in one table that refers to the primary key in another table.

It is used to create a relationship between two tables.

A foreign key can have duplicate values and can be NULL, depending on the design.

Example:
In a Courses table, if we have a column student_id that links to the Students table’s primary key, then student_id is a foreign key in the Courses table.