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 V1.73.2" /> 9 <link rel="start" href="index.html" title="Getting Started with Berkeley DB" /> 10 <link rel="up" href="indexes.html" title="Chapter��5.��Secondary Databases" /> 11 <link rel="prev" 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> 36 <p> 37 Like a primary database, you can read records from your secondary 38 database either by using the 39 40 <span> 41 <code class="methodname">DB->get()</code> 42 43 or 44 <code class="methodname">DB->pget()</code> 45 46 methods, 47 </span> 48 or by using 49 50 <span>a cursor on the secondary database.</span> 51 52 The main difference between reading secondary and primary databases is that when 53 you read a secondary database record, the secondary record's data is not 54 returned to you. Instead, the primary key and data corresponding to the 55 secondary key are returned to you. 56 </p> 57 <p> 58 For example, assuming your secondary database contains keys related 59 to a person's full name: 60 </p> 61 <a id="c_index6"></a> 62 <pre class="programlisting">#include <db.h> 63#include <string.h> 64 65... 66 67DB *my_secondary_database; 68DBT key; /* Used for the search key */ 69DBT pkey, pdata; /* Used to return the primary key and data */ 70char *search_name = "John Doe"; 71 72/* Primary and secondary database opens omitted for brevity */ 73 74/* Zero out the DBTs before using them. */ 75memset(&key, 0, sizeof(DBT)); 76memset(&pkey, 0, sizeof(DBT)); 77memset(&pdata, 0, sizeof(DBT)); 78 79key.data = search_name; 80key.size = strlen(search_name) + 1; 81 82/* Returns the key from the secondary database, and the data from the 83 * associated primary database entry. 84 */ 85my_secondary_database->get(my_secondary_database, NULL, 86 &key, &pdata, 0); 87 88/* Returns the key from the secondary database, and the key and data 89 * from the associated primary database entry. 90 */ 91my_secondary_database->pget(my_secondary_database, NULL, 92 &key, &pkey, &pdata, 0); </pre> 93 <p> 94 Note that, just like 95 96 <span>a primary database,</span> 97 98 if your secondary database supports duplicate records then 99 100 <span> 101 <code class="methodname">DB->get()</code> 102 103 and 104 <code class="methodname">DB->pget()</code> 105 106 </span> 107 only return the first record found in a matching duplicates set. If you 108 want to see all the records related to a specific secondary key, then use a 109 110 <span> 111 cursor opened on the secondary database. Cursors are described in 112 <a class="xref" href="Cursors.html" title="Chapter��4.��Using Cursors">Using Cursors</a>. 113 </span> 114 </p> 115 </div> 116 <div class="navfooter"> 117 <hr /> 118 <table width="100%" summary="Navigation footer"> 119 <tr> 120 <td width="40%" align="left"><a accesskey="p" href="keyCreator.html">Prev</a>��</td> 121 <td width="20%" align="center"> 122 <a accesskey="u" href="indexes.html">Up</a> 123 </td> 124 <td width="40%" align="right">��<a accesskey="n" href="secondaryDelete.html">Next</a></td> 125 </tr> 126 <tr> 127 <td width="40%" align="left" valign="top">Implementing Key 128 129 <span>Extractors</span> 130 ��</td> 131 <td width="20%" align="center"> 132 <a accesskey="h" href="index.html">Home</a> 133 </td> 134 <td width="40%" align="right" valign="top">��Deleting Secondary Database Records</td> 135 </tr> 136 </table> 137 </div> 138 </body> 139</html> 140