James F. Courtney and David B. Paradice's Database Systems for Management: "Chapter 6: The Relational Database Model"

Relational algebra is used to express a query as a sequence of basic operations which, when executed, will result in the relation that gives the query result. Relational algebra specifies 'how' to obtain the result. Relational calculus, on the other hand, is used to specify the desired result. This is 'what' the result is. The distinction is a difference of perspectives: relational algebra focuses on operations for constructing sequences of operations to produce a desired table, and relational calculus focuses on a relational expression that defines the desired table. Review pages 6–7 to 6–12 and study the case examples. Tables for the examples are included at the end of the chapter.