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No treatment of relational database technology would be complete without mention of structured<br />

query language (SQL). SQL is used to retrieve, insert, update, and delete data and to manage data<br />

(such as add tables, fields, functions, and views) in relational database management systems.<br />

Standards for SQL are overseen by the International Standards Organization (ISO) but many vendors<br />

of database management software add their own proprietary extensions (additional functionality<br />

and/or commands). SQL queries allow a user to specify the desired results and let the database<br />

management system determine how to physically perform the necessary operations to produce the<br />

results.<br />

Exhibit 20.7 Database data structures.<br />

The most common SQL operation is a query. A query starts with the SELECT keyword followed by<br />

a list of columns from tables (that are also specified as part of the query). Clauses can be added to<br />

specify criteria for inclusion, aggregation, or sorting. The SQL code for the template query shown in<br />

Exhibit 19.7 is as follows:

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