1<?xml version="1.0" encoding="UTF-8" standalone="no"?> 2<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> 3<html xmlns="http://www.w3.org/1999/xhtml"> 4 <head> 5 <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> 6 <title>Reading Secondary Databases</title> 7 <link rel="stylesheet" href="gettingStarted.css" type="text/css" /> 8 <meta name="generator" content="DocBook XSL Stylesheets V1.62.4" /> 9 <link rel="home" href="index.html" title="Getting Started with Berkeley DB" /> 10 <link rel="up" href="indexes.html" title="Chapter 5. Secondary Databases" /> 11 <link rel="previous" href="keyCreator.html" title="Implementing Key Extractors " /> 12 <link rel="next" href="secondaryDelete.html" title="Deleting Secondary Database Records" /> 13 </head> 14 <body> 15 <div class="navheader"> 16 <table width="100%" summary="Navigation header"> 17 <tr> 18 <th colspan="3" align="center">Reading Secondary Databases</th> 19 </tr> 20 <tr> 21 <td width="20%" align="left"><a accesskey="p" href="keyCreator.html">Prev</a> </td> 22 <th width="60%" align="center">Chapter 5. Secondary Databases</th> 23 <td width="20%" align="right"> <a accesskey="n" href="secondaryDelete.html">Next</a></td> 24 </tr> 25 </table> 26 <hr /> 27 </div> 28 <div class="sect1" lang="en" xml:lang="en"> 29 <div class="titlepage"> 30 <div> 31 <div> 32 <h2 class="title" style="clear: both"><a id="readSecondary"></a>Reading Secondary Databases</h2> 33 </div> 34 </div> 35 <div></div> 36 </div> 37 <p> 38 Like a primary database, you can read records from your secondary 39 database either by using the 40 41 <span> 42 43 <tt class="methodname">Db::get()</tt> 44 or 45 46 <tt class="methodname">Db::pget()</tt> 47 methods, 48 </span> 49 or by using 50 51 <span>a cursor on the secondary database.</span> 52 53 The main difference between reading secondary and primary databases is that when 54 you read a secondary database record, the secondary record's data is not 55 returned to you. Instead, the primary key and data corresponding to the 56 secondary key are returned to you. 57 </p> 58 <p> 59 For example, assuming your secondary database contains keys related 60 to a person's full name: 61 </p> 62 <a id="cxx_index6"></a> 63 <pre class="programlisting">#include <db_cxx.h> 64#include <string.h> 65 66... 67 68 69// The string to search for 70char *search_name = "John Doe"; 71 72// Instantiate our Dbt's 73Dbt key(search_name, strlen(search_name) + 1); 74Dbt pkey, pdata; // Primary key and data 75 76 77Db my_secondary_database(NULL, 0); 78// Primary and secondary database opens omitted for brevity 79 80// Returns the key from the secondary database, and the data from the 81// associated primary database entry. 82my_secondary_database.get(NULL, &key, &pdata, 0); 83 84// Returns the key from the secondary database, and the key and data 85// from the associated primary database entry. 86my_secondary_database.pget(NULL, &key, &pkey, &pdata, 0);</pre> 87 <p> 88 Note that, just like 89 90 <span>a primary database,</span> 91 92 if your secondary database supports duplicate records then 93 94 <span> 95 96 <tt class="methodname">Db::get()</tt> 97 and 98 99 <tt class="methodname">Db::pget()</tt> 100 </span> 101 only return the first record found in a matching duplicates set. If you 102 want to see all the records related to a specific secondary key, then use a 103 104 <span> 105 cursor opened on the secondary database. Cursors are described in 106 <a href="Cursors.html">Using Cursors</a>. 107 </span> 108 </p> 109 </div> 110 <div class="navfooter"> 111 <hr /> 112 <table width="100%" summary="Navigation footer"> 113 <tr> 114 <td width="40%" align="left"><a accesskey="p" href="keyCreator.html">Prev</a> </td> 115 <td width="20%" align="center"> 116 <a accesskey="u" href="indexes.html">Up</a> 117 </td> 118 <td width="40%" align="right"> <a accesskey="n" href="secondaryDelete.html">Next</a></td> 119 </tr> 120 <tr> 121 <td width="40%" align="left" valign="top">Implementing Key 122 123 Extractors 124 </td> 125 <td width="20%" align="center"> 126 <a accesskey="h" href="index.html">Home</a> 127 </td> 128 <td width="40%" align="right" valign="top"> Deleting Secondary Database Records</td> 129 </tr> 130 </table> 131 </div> 132 </body> 133</html> 134