Edit File: readSecondary.html
<?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>Reading Secondary 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="indexes.html" title="Chapter 5. Secondary Databases" /> <link rel="previous" href="keyCreator.html" title="Implementing Key Extractors " /> <link rel="next" href="secondaryDelete.html" title="Deleting Secondary Database Records" /> </head> <body> <div class="navheader"> <table width="100%" summary="Navigation header"> <tr> <th colspan="3" align="center">Reading Secondary Databases</th> </tr> <tr> <td width="20%" align="left"><a accesskey="p" href="keyCreator.html">Prev</a> </td> <th width="60%" align="center">Chapter 5. Secondary Databases</th> <td width="20%" align="right"> <a accesskey="n" href="secondaryDelete.html">Next</a></td> </tr> </table> <hr /> </div> <div class="sect1" lang="en" xml:lang="en"> <div class="titlepage"> <div> <div> <h2 class="title" style="clear: both"><a id="readSecondary"></a>Reading Secondary Databases</h2> </div> </div> <div></div> </div> <p> Like a primary database, you can read records from your secondary database either by using the <span> <tt class="methodname">DB->get()</tt> or <tt class="methodname">DB->pget()</tt> methods, </span> or by using <span>a cursor on the secondary database.</span> The main difference between reading secondary and primary databases is that when you read a secondary database record, the secondary record's data is not returned to you. Instead, the primary key and data corresponding to the secondary key are returned to you. </p> <p> For example, assuming your secondary database contains keys related to a person's full name: </p> <a id="c_index6"></a> <pre class="programlisting">#include <db.h> #include <string.h> ... DB *my_secondary_database; DBT key; /* Used for the search key */ DBT pkey, pdata; /* Used to return the primary key and data */ char *search_name = "John Doe"; /* Primary and secondary database opens omitted for brevity */ /* Zero out the DBTs before using them. */ memset(&key, 0, sizeof(DBT)); memset(&pkey, 0, sizeof(DBT)); memset(&pdata, 0, sizeof(DBT)); key.data = search_name; key.size = strlen(search_name) + 1; /* Returns the key from the secondary database, and the data from the * associated primary database entry. */ my_secondary_database->get(my_secondary_database, NULL, &key, &pdata, 0); /* Returns the key from the secondary database, and the key and data * from the associated primary database entry. */ my_secondary_database->pget(my_secondary_database, NULL, &key, &pkey, &pdata, 0); </pre> <p> Note that, just like <span>a primary database,</span> if your secondary database supports duplicate records then <span> <tt class="methodname">DB->get()</tt> and <tt class="methodname">DB->pget()</tt> </span> only return the first record found in a matching duplicates set. If you want to see all the records related to a specific secondary key, then use a <span> cursor opened on the secondary database. Cursors are described in <a href="Cursors.html">Using Cursors</a>. </span> </p> </div> <div class="navfooter"> <hr /> <table width="100%" summary="Navigation footer"> <tr> <td width="40%" align="left"><a accesskey="p" href="keyCreator.html">Prev</a> </td> <td width="20%" align="center"> <a accesskey="u" href="indexes.html">Up</a> </td> <td width="40%" align="right"> <a accesskey="n" href="secondaryDelete.html">Next</a></td> </tr> <tr> <td width="40%" align="left" valign="top">Implementing Key Extractors </td> <td width="20%" align="center"> <a accesskey="h" href="index.html">Home</a> </td> <td width="40%" align="right" valign="top"> Deleting Secondary Database Records</td> </tr> </table> </div> </body> </html>
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