Sql-Query

SQL: QUERIES

What men or gods are these? What maidens loth? What mad pursuit? What struggle to escape? What pipes and timbrels? What wild ecstasy? —John Keats, Ode on a Grecian Urn What is the average salary in the Toy department? —Anonymous SQL user

Structured Query Language (SQL) is the most widely used commercial relational database language. It was originally developed at IBM in the SEQUEL-XRM and System-R projects (1974–1977). Almost immediately, other vendors introduced DBMS products based on SQL, and it is now a de facto standard. SQL continues to evolve in response to changing needs in the database area. Our presentation follows the current ANSI/ISO standard for SQL, which is called SQL-92. We also discuss some important extensions in the new standard, SQL:1999. While not all DBMS products support the full SQL-92 standard yet, vendors are working toward this goal and most products already support the core features. The SQL language has several aspects to it: The Data Definition Language (DDL): This subset of SQL supports the creation, deletion, and modification of definitions for tables and views. Integrity constraints can be defined on tables, either when the table is created or later. The DDL also provides commands for specifying access rights or privileges to tables and views. Although the standard does not discuss indexes, commercial implementations also provide commands for creating and deleting indexes. We covered the DDL features of SQL in Chapter 3. The Data Manipulation Language (DML): This subset of SQL allows users to pose queries and to insert, delete, and modify rows. We covered DML commands to insert, delete, and modify rows in Chapter 3. Embedded and dynamic SQL: Embedded SQL features allow SQL code to be called from a host language such as C or COBOL. Dynamic SQL features allow a query to be constructed (and executed) at run-time. Triggers: The new SQL:1999 standard includes support for triggers, which are actions executed by the DBMS...