If you need an ER diagram for an online shopping system, look no further. In this article, you will get a complete database model for online shopping as well as a step-by-step guide to building it. We’ll also talk about some ideas to extend the model. An ER diagram is a blueprint for creating the foundations of any application that uses information stored in databases. Data modelers use the ER diagrams like architects use their drawing boards: on an empty canvas, they place the elements that make up the structure of what they are going to build.