11 – Relational Languages

Publisher Summary The relational schema may be implemented in a relational database management system (RDBMS) that has plenty of data. 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 the relational assignment. A relational system that supports all eight table operations of the relational algebra is said to be relationally complete. The two main relational languages are SQL and QBE, with SQL being the most important. Most SQL systems are relationally complete. A system that supports all aspects of the relational model, including domains and the two basic integrity rules (entity integrity and referential integrity), is said to be “fully relational.” The relational model of data itself is evolving. In addition to relational algebra operations, relational DBMSs provide further capabilities such as sorting, arithmetic, grouping, and formatting. Most systems also provide powerful tools for creating external interfaces, report writing, and security.