The Sample Database

The Sample Database

Most of the examples in this course use two small tables you should memorize: Customers and Orders.

The Customers table

• CustomerID is the primary key - every customer gets a unique number.
• CustomerName and ContactName are text.
• Country is where the customer lives.

The Orders table

• OrderID is the primary key for orders.
• CustomerID links each order to a customer - this is a foreign key.
• Customer 1 (Alfreds) has two orders here.

How the tables connect

Customers.CustomerID  <-->  Orders.CustomerID

This relationship is what makes joins possible - more on that later.

Primary key vs foreign key

• Primary key: a column (or columns) that uniquely identifies each row in its own table.
• Foreign key: a column that points to the primary key of another table.

TL;DR

• Customers: CustomerID, CustomerName, ContactName, Country.
• Orders: OrderID, CustomerID, OrderDate.
• Orders.CustomerID references Customers.CustomerID.
• Primary keys identify rows; foreign keys link tables.