This chapter presents the relational model and three relational languages. The rela- tional model (Section ) is used extensively throughout the text as is the relational algebra (Section ). The chapter also covers the tuple relational calculus (Section ) and domain relational calculus (Section ) (which is the basis of the QBE language described in Chapter 5). Classes that emphasize only SQL may omit the relational calculus languages. Our notation for the tuple relational calculus makes it easy to present the con- cept of a safe query. The concept of safety for the domain relational calculus, though identical to that for the tuple calculus, is much more cumbersome.
