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 <tt class="methodname">DB->get()</tt> 43 44 or 45 <tt class="methodname">DB->pget()</tt> 46 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="c_index6"></a> 63 <pre class="programlisting">#include <db.h> 64#include <string.h> 65 66... 67 68DB *my_secondary_database; 69DBT key; /* Used for the search key */ 70DBT pkey, pdata; /* Used to return the primary key and data */ 71char *search_name = "John Doe"; 72 73/* Primary and secondary database opens omitted for brevity */ 74 75/* Zero out the DBTs before using them. */ 76memset(&key, 0, sizeof(DBT)); 77memset(&pkey, 0, sizeof(DBT)); 78memset(&pdata, 0, sizeof(DBT)); 79 80key.data = search_name; 81key.size = strlen(search_name) + 1; 82 83/* Returns the key from the secondary database, and the data from the 84 * associated primary database entry. 85 */ 86my_secondary_database->get(my_secondary_database, NULL, 87 &key, &pdata, 0); 88 89/* Returns the key from the secondary database, and the key and data 90 * from the associated primary database entry. 91 */ 92my_secondary_database->pget(my_secondary_database, NULL, 93 &key, &pkey, &pdata, 0); </pre> 94 <p> 95 Note that, just like 96 97 <span>a primary database,</span> 98 99 if your secondary database supports duplicate records then 100 101 <span> 102 <tt class="methodname">DB->get()</tt> 103 104 and 105 <tt class="methodname">DB->pget()</tt> 106 107 </span> 108 only return the first record found in a matching duplicates set. If you 109 want to see all the records related to a specific secondary key, then use a 110 111 <span> 112 cursor opened on the secondary database. Cursors are described in 113 <a href="Cursors.html">Using Cursors</a>. 114 </span> 115 </p> 116 </div> 117 <div class="navfooter"> 118 <hr /> 119 <table width="100%" summary="Navigation footer"> 120 <tr> 121 <td width="40%" align="left"><a accesskey="p" href="keyCreator.html">Prev</a>��</td> 122 <td width="20%" align="center"> 123 <a accesskey="u" href="indexes.html">Up</a> 124 </td> 125 <td width="40%" align="right">��<a accesskey="n" href="secondaryDelete.html">Next</a></td> 126 </tr> 127 <tr> 128 <td width="40%" align="left" valign="top">Implementing Key 129 130 Extractors 131 ��</td> 132 <td width="20%" align="center"> 133 <a accesskey="h" href="index.html">Home</a> 134 </td> 135 <td width="40%" align="right" valign="top">��Deleting Secondary Database Records</td> 136 </tr> 137 </table> 138 </div> 139 </body> 140</html> 141