TABLE OF CONTENTS 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 belt, we will be able define what is really meant by the term relational database system . The following sections then cover SQL in some depth, starting with the basics and moving on through intermediate to advanced concepts. Although our focus is on the SQL standard(s), some additional detail will be included about commercial dialects of SQL. Now let s begin with relational algebra.
In the relational model of data, all facts are stored in tables (or relations). New tables may be formed from existing tables by applying operations in the relational algebra. The tables resulting from these operations may be named and stored, using relational assignment. The original relational algebra defined by Codd contained eight relational operators: four based on traditional set operations (union, intersection, difference, and Cartesian product)...
