计算机科学与技术网上购书系统毕业设计.doc

计算机科学与技术网上购书系统毕业设计 1 SQL The formal languages described in Chapter 3 provide a concise notation for representing queries. However, commercial database systems require a query language that is more users friendly. In this chapter, we study SQL, the most influential commercially marketed query language, SQL. SQL uses a combination of relational-algebra and relational-calculus constructs. Although we refer to the SQL language as a “query language,” it can do much more than just query a database. It can define the structure of the data, modify data in the database, and specify security constraints. It is not our intention to provide a complete users’ guide for SQL. Rather, we present SQLs fundamental constructs and concepts. Individual implementations of SQL may differ in details, or may support onl11a subset of the full language. 1.1 Background IBM developed the original version of SQL at its San Jose Research Laboratory (now the Almaden Research Center). IBM implemented the language, originally called Sequel, as part of the System R project in the early 1970s. The Sequel language has evolved since then, and its name has changed to SQL (Structured Query Language). Many products now support the SQL language. SQL has clearly established itself as the standard relational-database language. In 1986, the American National Standards Institute (ANSI) and the International Organization for Standardization (ISO) published an SQL standard, called SQL-86. IBM published its own corporate SQL standard, the Systems Application Architecture Database Interface (SAA-SQL) in 1987. ANSI published an extended standard for SQL, SQL-89, in 1989. The next version of the standard was SQL-92 standard, and the most recent version is SQL: 1999. The SQL language has several parts: Data-definition language (DDL). The SQL DDL provides commands for defining relation schemas, deleting relations, and modifying relation schemas. Interactive data-manipulation language (DML). The SQL DML includes a que