Introduction to Structured Query Language
This page is a introductory tutorial of the Structured Query Language (also known as SQL) and is a pioneering effort on the World Wide Web, as this is the first comprehensive SQL tutorial available on the Internet. SQL allows users to access data in relational database management systems, such as Oracle, Sybase, Informix, Microsoft SQL Server, Access, and others, by allowing users to describe the data the user wishes to see. SQL also allows users to define the data in a database, and manipulate that data. This page will describe how to use SQL, and give examples. The SQL used in this document is “ANSI”, or standard SQL, and no SQL features of specific database management systems will be discussed until the “Nonstandard SQL” section. It is recommended that you print this page, so that you can easily refer back to previous examples.
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To generalize this process, SQL performs the AND operation(s) to determine the rows where the AND operation(s) hold true (remember: all of the conditions are true), then these results are used to compare with the OR conditions, and only display those remaining rows where any of the conditions joined by the OR operator hold true (where a condition or result from an AND is paired with another condition or AND result to use to evaluate the OR, which evaluates to true if either value is true). Mathematically, SQL evaluates all of the conditions, then evaluates the AND “pairs”, and then evaluates the OR’s (where both operators evaluate left to right). To look at an example, for a given row for which the DBMS is evaluating the SQL statement Where clause to determine whether to include the row in the query result (the whole Where clause evaluates to True), the DBMS has evaluated all of the conditions, and is ready to do the logical comparisons on this result:
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