141 lines
5.5 KiB
HTML
141 lines
5.5 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>Reading Secondary Databases</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="keyCreator.html" title="Implementing Key Extractors " />
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<link rel="next" href="secondaryDelete.html" title="Deleting Secondary Database Records" />
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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">Reading Secondary Databases</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="keyCreator.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="secondaryDelete.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="readSecondary"></a>Reading Secondary Databases</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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Like a primary database, you can read records from your secondary
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database either by using the
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<span>
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<tt class="methodname">DB->get()</tt>
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or
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<tt class="methodname">DB->pget()</tt>
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methods,
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</span>
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or by using
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<span>a cursor on the secondary database.</span>
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The main difference between reading secondary and primary databases is that when
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you read a secondary database record, the secondary record's data is not
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returned to you. Instead, the primary key and data corresponding to the
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secondary key are returned to you.
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</p>
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<p>
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For example, assuming your secondary database contains keys related
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to a person's full name:
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</p>
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<a id="c_index6"></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 *my_secondary_database;
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DBT key; /* Used for the search key */
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DBT pkey, pdata; /* Used to return the primary key and data */
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char *search_name = "John Doe";
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/* Primary and secondary database opens omitted for brevity */
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/* Zero out the DBTs before using them. */
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memset(&key, 0, sizeof(DBT));
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memset(&pkey, 0, sizeof(DBT));
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memset(&pdata, 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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/* Returns the key from the secondary database, and the data from the
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* associated primary database entry.
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*/
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my_secondary_database->get(my_secondary_database, NULL,
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&key, &pdata, 0);
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/* Returns the key from the secondary database, and the key and data
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* from the associated primary database entry.
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*/
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my_secondary_database->pget(my_secondary_database, NULL,
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&key, &pkey, &pdata, 0); </pre>
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<p>
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Note that, just like
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<span>a primary database,</span>
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if your secondary database supports duplicate records then
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<span>
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<tt class="methodname">DB->get()</tt>
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and
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<tt class="methodname">DB->pget()</tt>
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</span>
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only return the first record found in a matching duplicates set. If you
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want to see all the records related to a specific secondary key, then use a
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<span>
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cursor opened on the secondary database. Cursors are described in
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<a href="Cursors.html">Using Cursors</a>.
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</span>
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</p>
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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="keyCreator.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="secondaryDelete.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">Implementing Key
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Extractors
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</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"> Deleting Secondary Database Records</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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