12.1. SQL: Relational Algebra
So far we’ve seen how to design a conceptual schema and map it to a relational schema. The relational schema may now be implemented in a relational DBMS, and its tables populated with data. To retrieve information from the resulting database, we need to issue queries. In practice the most popular query languages are SQL (popularly, if incorrectly, called “Structured Query Language”) and QBE (Query By Example). Both of these are based, at least partly, on a formal query language known as relational algebra, which is discussed in this section. Studying this algebra first clarifies the basic query operations without getting distracted by the specific syntax of commercial query languages.
With the algebra under our ...
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