<!--$Id: keydata.so,v 10.19 2000/12/14 21:42:18 bostic Exp $--> <!--Copyright 1997-2003 by Sleepycat Software, Inc.--> <!--All rights reserved.--> <!--See the file LICENSE for redistribution information.--> <html> <head> <title>Berkeley DB Reference Guide: Key/data pairs</title> <meta name="description" content="Berkeley DB: An embedded database programmatic toolkit."> <meta name="keywords" content="embedded,database,programmatic,toolkit,b+tree,btree,hash,hashing,transaction,transactions,locking,logging,access method,access methods,Java,C,C++"> </head> <body bgcolor=white> <table width="100%"><tr valign=top> <td><h3><dl><dt>Berkeley DB Reference Guide:<dd>Simple Tutorial</dl></h3></td> <td align=right><a href="../simple_tut/intro.html"><img src="../../images/prev.gif" alt="Prev"></a><a href="../toc.html"><img src="../../images/ref.gif" alt="Ref"></a><a href="../simple_tut/handles.html"><img src="../../images/next.gif" alt="Next"></a> </td></tr></table> <p> <h3 align=center>Key/data pairs</h3> <p>Berkeley DB uses key/data pairs to identify elements in the database. That is, in the general case, whenever you call a Berkeley DB interface, you present a key to identify the key/data pair on which you intend to operate.</p> <p>For example, you might store some key/data pairs as follows:</p> <table border=1> <tr><th>Key:</th><th>Data:</th></tr> <tr><td>fruit</td><td>apple</td></tr> <tr><td>sport</td><td>cricket</td></tr> <tr><td>drink</td><td>water</td></tr> </table> <p>In each case, the first element of the pair is the key, and the second is the data. To store the first of these key/data pairs into the database, you would call the Berkeley DB interface to store items, with <b>fruit</b> as the key, and <b>apple</b> as the data. At some future time, you could then retrieve the data item associated with <b>fruit</b>, and the Berkeley DB retrieval interface would return <b>apple</b> to you. While there are many variations and some subtleties, all accesses to data in Berkeley DB come down to key/data pairs.</p> <p>Both key and data items are stored in simple structures (called <a href="../../api_c/dbt_class.html">DBT</a>s) that contain a reference to memory and a length, counted in bytes. (The name <a href="../../api_c/dbt_class.html">DBT</a> is an acronym for <i>database thang</i>, chosen because nobody could think of a sensible name that wasn't already in use somewhere else.) Key and data items can be arbitrary binary data of practically any length, including 0 bytes. There is a single data item for each key item, by default, but databases can be configured to support multiple data items for each key item.</p> <table width="100%"><tr><td><br></td><td align=right><a href="../simple_tut/intro.html"><img src="../../images/prev.gif" alt="Prev"></a><a href="../toc.html"><img src="../../images/ref.gif" alt="Ref"></a><a href="../simple_tut/handles.html"><img src="../../images/next.gif" alt="Next"></a> </td></tr></table> <p><font size=1><a href="../../sleepycat/legal.html">Copyright (c) 1996-2003</a> <a href="http://www.sleepycat.com">Sleepycat Software, Inc.</a> - All rights reserved.</font> </body> </html>
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