158 lines
5.7 KiB
HTML
158 lines
5.7 KiB
HTML
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml">
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
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<title>
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Using Cursors with Secondary Databases
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</title>
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<link rel="stylesheet" href="gettingStarted.css" type="text/css" />
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<meta name="generator" content="DocBook XSL Stylesheets V1.62.4" />
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<link rel="home" href="index.html" title="Getting Started with Berkeley DB" />
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<link rel="up" href="indexes.html" title="Chapter 5. Secondary Databases" />
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<link rel="previous" href="secondaryDelete.html" title="Deleting Secondary Database Records" />
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<link rel="next" href="joins.html" title="Database Joins" />
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</head>
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<body>
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<div class="navheader">
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<table width="100%" summary="Navigation header">
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<tr>
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<th colspan="3" align="center">
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Using Cursors with Secondary Databases
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</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="secondaryDelete.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 5. Secondary Databases</th>
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<td width="20%" align="right"> <a accesskey="n" href="joins.html">Next</a></td>
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</tr>
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</table>
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<hr />
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</div>
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<div class="sect1" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h2 class="title" style="clear: both"><a id="secondaryCursor"></a>
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<span>Using Cursors with Secondary Databases</span>
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</h2>
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</div>
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</div>
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<div></div>
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</div>
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<p>
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Just like cursors on a primary database, you can use
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<span>cursors on secondary databases</span>
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to iterate over the records in a secondary database. Like
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<span>cursors used with primary databases,</span>
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you can also use
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<span>cursors with secondary databases</span>
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to search for specific records in a database, to seek to the first
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or last record in the database, to get the next duplicate record,
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and so forth. For a complete description on cursors and their capabilities, see
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<a href="Cursors.html">Using Cursors</a>.
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</p>
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<p>
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However, when you use
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<span>cursors with secondary databases:</span>
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</p>
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<div class="itemizedlist">
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<ul type="disc">
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<li>
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<p>
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Any data returned is the data contained on the primary database
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record referenced by the secondary record.
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</p>
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</li>
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<li>
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<p>
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You cannot use <tt class="literal">DB_GET_BOTH</tt> and related flags with
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<tt class="methodname">DB->get()</tt>
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and a secondary database. Instead, you must use
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<span><tt class="methodname">DB->pget()</tt>.</span>
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Also, in that case the primary and secondary key given on the call to
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<tt class="methodname">DB->pget()</tt>
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must match the secondary key and associated primary record key in
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order for that primary record to be returned as a result of the
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call.
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</p>
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</li>
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</ul>
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</div>
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<p>
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For example, suppose you are using the databases, classes, and key
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<span>extractors</span>
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described in <a href="keyCreator.html">Implementing Key
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<span>Extractors</span>
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</a>.
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Then the following searches for a person's
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name in the secondary database, and deletes all secondary and primary
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records that use that name.
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</p>
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<a id="c_index8"></a>
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<pre class="programlisting">#include <db.h>
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#include <string.h>
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...
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DB *sdbp; /* Secondary DB handle */
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DBC *cursorp; /* Cursor */
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DBT key, data; /* DBTs used for the delete */
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char *search_name = "John Doe"; /* Name to delete */
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/* Primary and secondary database opens omitted for brevity. */
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/* Get a cursor on the secondary database */
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sdbp->cursor(sdbp, NULL, &cursorp, 0);
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/*
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* Zero out the DBT before using it.
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*/
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memset(&key, 0, sizeof(DBT));
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memset(&data, 0, sizeof(DBT));
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key.data = search_name;
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key.size = strlen(search_name) + 1;
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/* Position the cursor */
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while (cursorp->get(cursorp, &key, &data, DB_SET) == 0)
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cursorp->del(cursorp, 0); </pre>
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</div>
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<div class="navfooter">
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<hr />
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<table width="100%" summary="Navigation footer">
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<tr>
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<td width="40%" align="left"><a accesskey="p" href="secondaryDelete.html">Prev</a> </td>
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<td width="20%" align="center">
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<a accesskey="u" href="indexes.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="joins.html">Next</a></td>
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</tr>
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<tr>
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<td width="40%" align="left" valign="top">Deleting Secondary Database Records </td>
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<td width="20%" align="center">
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<a accesskey="h" href="index.html">Home</a>
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</td>
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<td width="40%" align="right" valign="top"> Database Joins</td>
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</tr>
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</table>
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</div>
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</body>
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</html>
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