<?xml version="1.0" encoding="UTF-8" standalone="no"?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> <title>Chapter 2. Databases</title> <link rel="stylesheet" href="gettingStarted.css" type="text/css" /> <meta name="generator" content="DocBook XSL Stylesheets V1.62.4" /> <link rel="home" href="index.html" title="Getting Started with Berkeley DB" /> <link rel="up" href="index.html" title="Getting Started with Berkeley DB" /> <link rel="previous" href="gettingit.html" title="Getting and Using DB " /> <link rel="next" href="coredbclose.html" title="Closing Databases" /> </head> <body> <div class="navheader"> <table width="100%" summary="Navigation header"> <tr> <th colspan="3" align="center">Chapter 2. Databases</th> </tr> <tr> <td width="20%" align="left"><a accesskey="p" href="gettingit.html">Prev</a> </td> <th width="60%" align="center"> </th> <td width="20%" align="right"> <a accesskey="n" href="coredbclose.html">Next</a></td> </tr> </table> <hr /> </div> <div class="chapter" lang="en" xml:lang="en"> <div class="titlepage"> <div> <div> <h2 class="title"><a id="DB"></a>Chapter 2. Databases</h2> </div> </div> <div></div> </div> <div class="toc"> <p> <b>Table of Contents</b> </p> <dl> <dt> <span class="sect1"> <a href="DB.html#DBOpen">Opening Databases</a> </span> </dt> <dt> <span class="sect1"> <a href="coredbclose.html">Closing Databases</a> </span> </dt> <dt> <span class="sect1"> <a href="DBOpenFlags.html">Database Open Flags</a> </span> </dt> <dt> <span class="sect1"> <a href="CoreDBAdmin.html">Administrative Methods</a> </span> </dt> <dt> <span class="sect1"> <a href="dbErrorReporting.html">Error Reporting Functions</a> </span> </dt> <dt> <span class="sect1"> <a href="CoreEnvUsage.html">Managing Databases in Environments</a> </span> </dt> <dt> <span class="sect1"> <a href="CoreDbCXXUsage.html">Database Example</a> </span> </dt> </dl> </div> <p>In Berkeley DB, a database is a collection of <span class="emphasis"><em>records</em></span>. Records, in turn, consist of key/data pairings. </p> <p> Conceptually, you can think of a <span>database</span> as containing a two-column table where column 1 contains a key and column 2 contains data. Both the key and the data are managed using <tt class="classname">Dbt</tt> <span>class instances</span> (see <a href="DBEntry.html">Database Records</a> for details on this <span>class</span> ). So, fundamentally, using a DB <span>database</span> involves putting, getting, and deleting database records, which in turns involves efficiently managing information <span>encapsulated by </span> <tt class="classname">Dbt</tt> <span>objects.</span> The next several chapters of this book are dedicated to those activities. </p> <div class="sect1" lang="en" xml:lang="en"> <div class="titlepage"> <div> <div> <h2 class="title" style="clear: both"><a id="DBOpen"></a>Opening Databases</h2> </div> </div> <div></div> </div> <p> You open a database by instantiating a <tt class="classname">Db</tt> object and then calling its <tt class="methodname">open()</tt> method. </p> <p> Note that by default, DB does not create databases if they do not already exist. To override this behavior, specify the <a href="DBOpenFlags.html" title="Database Open Flags"><tt class="literal">DB_CREATE</tt></a> flag on the <tt class="methodname">open()</tt> method. </p> <p> The following code fragment illustrates a database open: </p> <a id="cxx_db1"></a> <pre class="programlisting">#include <db_cxx.h> ... Db db(NULL, 0); // Instantiate the Db object u_int32_t oFlags = DB_CREATE; // Open flags; try { // Open the database db.open(NULL, // Transaction pointer "my_db.db", // Database file name NULL, // Optional logical database name DB_BTREE, // Database access method oFlags, // Open flags 0); // File mode (using defaults) // DbException is not subclassed from std::exception, so // need to catch both of these. } catch(DbException &e) { // Error handling code goes here } catch(std::exception &e) { // Error handling code goes here } </pre> </div> </div> <div class="navfooter"> <hr /> <table width="100%" summary="Navigation footer"> <tr> <td width="40%" align="left"><a accesskey="p" href="gettingit.html">Prev</a> </td> <td width="20%" align="center"> <a accesskey="u" href="index.html">Up</a> </td> <td width="40%" align="right"> <a accesskey="n" href="coredbclose.html">Next</a></td> </tr> <tr> <td width="40%" align="left" valign="top">Getting and Using DB </td> <td width="20%" align="center"> <a accesskey="h" href="index.html">Home</a> </td> <td width="40%" align="right" valign="top"> Closing Databases</td> </tr> </table> </div> </body> </html>