This tutorial covers what many people agree is one of the most difficult database conceptshow to create and work with . A relational database contains two or more tables that are related to each other in some way.For example, a database might contain a Customers table and an Invoices table that contains the customer's orders. In this tutorial you will learn how to link tables in an existing database together in a one-to-many relationship to create a relational database. You will also learn how to enforce referential integrity between those tables to keep records in related fields valid and accurate.