The Entity–Relationship (ER) model is one such example. ER modeling is a top-down approach to database design that begins by identifying the important data called entities and relationships between the data that must be represented in the model. We then add more details such as the information we want to hold about the entities and relation- ships called attributes and any constraints on the entities, relationships, and attributes. ER modeling is an important technique for any database designer to master and forms the basis of the methodology presented in this book.