Encyclopedia of Computer Science and Technology

SQL 455Structured Query Language (SQL) is a standardized way to query and manipulate databases. Here the statement SELECT NUMBER, NAME,PRICE WHERE PRICE >= 50.00 extracts only the records meeting that criterion.franca for relational database systems (see database managementsystem).A relational database (such as Oracle, Sybase, IBM DB2,and Microsoft Access) stores data in tables called relations.The columns in the table describe the characteristics ofan entity (corresponding to data fields). For example, in acustomer database the Customer table might include attributessuch as customer number, First_name, Last_Name,Street, City, Phone_number, and so on. The rows in thetable (sometimes called tuples) represent the data recordsfor the various customers.Many database systems have more than one table. Forexample, a store’s database might contain a Customers table(for information identifying a customer), an Item table (givingcharacteristics of an item, such as price and number instock), and a Transaction table (whose characteristics mightbe customer number, date, item bought, and so on). Noticethat the Transaction record contains both a customer numberand an item number. It thus serves as a sort of bridge orlink between the Customer and Item tables.SQL provides commands that can be used to specify andaccess components of a database. For example, the INSERTand DELETE commands can be used to add or remove rows(records) from tables.To query a database means to give criteria for selectingcertain records from a table. For example, the querySELECT * FROM CUSTOMERS WHERE LAST_NAME =“Howard”would return the complete records for all customers whoselast name is Howard. If only selected fields are desired, theycan be specified like this:SELECT NUMBER, NAME, PRICE FROM ITEMS WHEREPRICE > = 50.00This query will display the Number, Name, and Price fieldsfor all items whose price is greater than or equal to $50.00.SQL includes many commands to further refine dataprocessing and reporting. There are built-in mathematicalfunctions as well as a GROUP BY command for furtherbreaking down a report by a particular field name orvalue.SQL can be used interactively by typing commands ata prompt, but database applications designed for less technicalusers often provide a user-friendly query form (andperhaps menus or buttons). After the user selects the appropriatefields and values, the program will then generate thenecessary SQL statements and send them to the internal“database engine” for processing. The results will then bedisplayed for the user.SQL procedures can be stored and managed as part ofa database. SQL can also be “embedded” within a morecomplete programming language environment so that, forexample, a Java program can perform SQL operations whileusing Java for processing that cannot be specified in SQL.In the mid-1990s an object-oriented version of SQL calledOQL (object query language), allowing the use of that popularparadigm for database operations (see object-orientedprogramming).One of the most popular implementations of SQL isMySQL, which is privately owned and developed but availablefor free license on many platforms, including Windowsand Linux. A number of applications are designed to workwith MySQL databases: see, for example, wikis and Wikipediaand YouTube.Further ReadingForta, Ben. Sams Teach Yourself SQL in 10 Minutes. 3rd ed. Indianapolis:Sams, 2004.Kofler, Michael. The Definitive Guide to MySQL 5. 3rd ed. Berkeley,Calif.: Apress, 2005.MySQL home page. Available online. URL: http://mysql.org/.Accessed August 22, 2007.Rankins, Ray, et al. Microsoft SQL Server 2005 Unleashed. Indianapolis:Sams, 2006.Tahaghoghi, Seyed M. M., and Hugh Williams. Learning MySQL.Sebastapol, Calif.: O’Reilly Media, 2006.Taylor, Allen G. SQL for Dummies. 6th ed. Hoboken, N.J.: Wiley,2006.