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Data models that preceded the relational model include the hierarchical and network models. They were proposed in the 1960s and were implemented in early DBMSs during the late 1960s and early 1970s. Because of their historical importance and the existing user base for these DBMSs, we have included a summary of the highlights of these models in Appendices D and E, which are available on this book’s Companion Website at http://www.aw.com/elmasri. These models and systems are now referred to as legacy database systems. In this chapter, we concentrate on describing the basic principles of the relational model of data. We begin by defining the modeling concepts and notation of the relational model in Section 3.1. Section 3.2 is devoted to a discussion of relational constraints that are considered an important part of the relational model and are automatically enforced in most relational DBMSs. Section 3.3 defines the update operations of the relational model,discusses how violations of integrity constraints are handled, and introduces the concept of a transaction. Section 3.4 summarizes the chapter.