## Primary key and Foreign key
**Relational databases** are based on tables and relationships, which are implemented using the **Primary Key** and the **Foreign Key**.

- A **Primary Key (PK)** is a unique identifier of the record in the table.

- A **Foreign Key (FK)** is a reference from a record in one table to the **Primary Key** of another table.

For example, consider two tables: “**student**” and “**group**”. Each **student** record includes data such as **id** (student id), **name**, **surname**, **age**, and **group_id** (which is a foreign key referencing the primary key of the “**group**” table). The “**group**” table includes data such as **id** (group id), **name**, and **grade**.

Now we can see that **John** and **Kesha** belong to group **BG1** (they have a foreign key reference to **group id**), group **BG4** has one student, **Emma**, and group **MG2** has no students.

This structure allows for the execution of `JOIN` and other operations.

Look at the “**group-student**” database entity-relationship (ER) diagram:

In this section, you will become familiar with both relational and non-relational databases and learn how to create diagrams to represent database structures.

This section delves deeper into the topic of relational databases and provides a comprehensive overview of the key theoretical concepts. Topics covered include foreign keys, the different ON DELETE options, and the various types of relationships.

It's time to learn how to construct databases, with a focus on ensuring a well-structured database for optimal functionality. In this section, you will familiarize yourself with the key principles for building a database for ease of use.

Primary key and Foreign key

What is a primary key?

What is a foreign key?