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194 lines
7.3 KiB
HTML
194 lines
7.3 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>Join Cursors</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 V1.73.2" />
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<link rel="start" href="index.html" title="Getting Started with Berkeley DB Java Edition" />
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<link rel="up" href="persist_access.html" title="Chapter 5. Saving and Retrieving Objects" />
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<link rel="prev" href="getmultiple.html" title="Retrieving Multiple Objects" />
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<link rel="next" href="dpl_delete.html" title="Deleting Entity Objects" />
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</head>
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<body>
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<div xmlns="" class="navheader">
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<div class="libver">
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<p>Library Version 12.2.7.5</p>
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</div>
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<table width="100%" summary="Navigation header">
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<tr>
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<th colspan="3" align="center">Join Cursors</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="getmultiple.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 5. Saving and Retrieving Objects</th>
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<td width="20%" align="right"> <a accesskey="n" href="dpl_delete.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="dpl_entityjoin"></a>Join Cursors</h2>
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</div>
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</div>
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</div>
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<p>
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If you have two or more secondary indexes set for
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an entity object, then you can retrieve sets of
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objects based on the intersection of multiple
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secondary index values. You do this using an
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<code class="classname">EntityJoin</code>
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class.
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</p>
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<p>
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For example, suppose you had an entity class that
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represented automobiles. In that case, you might
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be storing information about automobiles such as
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color, number of doors, fuel mileage,
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automobile type, number of passengers, make, model, and year,
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to name just a few.
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</p>
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<p>
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If you created a secondary index based this
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information, then you could use an
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<code class="classname">EntityJoin</code> to return
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all those objects representing cars with, say, two
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doors, that were built in 2002, and which are green
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in color.
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</p>
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<p>
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To create a join cursor, you:
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</p>
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<div class="orderedlist">
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<ol type="1">
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<li>
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<p>
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Open the primary index for the
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entity class on which you want to
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perform the join.
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</p>
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</li>
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<li>
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<p>
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Open the secondary indexes that you
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want to use for the join.
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</p>
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</li>
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<li>
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<p>
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Instantiate an
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<code class="classname">EntityJoin</code>
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object (you use the primary index
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to do this).
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</p>
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</li>
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<li>
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<p>
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Use two or more calls to
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<code class="methodname">EntityJoin.addCondition()</code>
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to identify the secondary indexes
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and their values that you want to use
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for the equality match.
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</p>
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</li>
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<li>
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<p>
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Call
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<code class="methodname">EntityJoin.entities()</code>
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to obtain a cursor that you can use
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to iterate over the join results.
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</p>
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</li>
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</ol>
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</div>
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<p>
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For example, suppose we had an entity class
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that included the following features:
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</p>
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<pre class="programlisting">package persist.gettingStarted;
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import com.sleepycat.persist.model.Entity;
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import com.sleepycat.persist.model.PrimaryKey;
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import static com.sleepycat.persist.model.Relationship.*;
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import com.sleepycat.persist.model.SecondaryKey;
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@Entity
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public class Automobiles {
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// Primary key is the vehicle identification number
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@PrimaryKey
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private String vin;
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// Secondary key is the vehicle's make
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@SecondaryKey(relate=MANY_TO_ONE)
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private String make;
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// Secondary key is the vehicle's color
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@SecondaryKey(relate=MANY_TO_ONE)
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private String color;
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...
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public String getVIN() {
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return vin;
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}
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public String getMake() {
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return make;
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}
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public String getColor() {
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return color;
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}
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... </pre>
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<p>
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Then we could perform an entity join that searches for all the
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red automobiles made by Toyota as follows:
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</p>
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<pre class="programlisting">
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PrimaryIndex<String,Automobiles> vin_pidx;
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SecondaryIndex<String,String,Automobiles> make_sidx;
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SecondaryIndex<String,String,Automobiles> color_sidx;
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EntityJoin<String,Automobiles> join = new EntityJoin(vin_pidx);
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join.addCondition(make_sidx,"Toyota");
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join.addCondition(color_sidx,"Red");
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// Now iterate over the results of the join operation
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ForwardCursor<Automobiles> join_cursor = join.entities();
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try {
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for (Automobiles autoi : join_cursor) {
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// do something with each object "autoi"
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}
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// Always make sure the cursor is closed when we are done with it.
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} finally {
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join_cursor.close();
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} </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="getmultiple.html">Prev</a> </td>
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||
<td width="20%" align="center">
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||
<a accesskey="u" href="persist_access.html">Up</a>
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||
</td>
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||
<td width="40%" align="right"> <a accesskey="n" href="dpl_delete.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">Retrieving Multiple Objects </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 Entity Objects</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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