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| author | jbj <devnull@localhost> | 2004-10-16 01:31:54 +0000 |
|---|---|---|
| committer | jbj <devnull@localhost> | 2004-10-16 01:31:54 +0000 |
| commit | d03f220fde879509cab2ac1c73b71b7efb52b737 (patch) | |
| tree | 1e34bfadac0a6618d0e9a7933bad90063a785acf /db/docs/gsg/JAVA/joins.html | |
| parent | 2dc699bfe049b9319ea3719f604d25940ff52004 (diff) | |
| download | librpm-tizen-d03f220fde879509cab2ac1c73b71b7efb52b737.tar.gz librpm-tizen-d03f220fde879509cab2ac1c73b71b7efb52b737.tar.bz2 librpm-tizen-d03f220fde879509cab2ac1c73b71b7efb52b737.zip | |
... and in with the New ...
CVS patchset: 7471
CVS date: 2004/10/16 01:31:54
Diffstat (limited to 'db/docs/gsg/JAVA/joins.html')
| -rw-r--r-- | db/docs/gsg/JAVA/joins.html | 366 |
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diff --git a/db/docs/gsg/JAVA/joins.html b/db/docs/gsg/JAVA/joins.html new file mode 100644 index 000000000..a22619540 --- /dev/null +++ b/db/docs/gsg/JAVA/joins.html @@ -0,0 +1,366 @@ +<?xml version="1.0" encoding="ISO-8859-1" 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=ISO-8859-1" /> + <title>Database Joins</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="secondaryCursor.html" title=" Using Secondary Cursors " /> + <link rel="next" href="javaindexusage.html" title="Secondary Database Example" /> + </head> + <body> + <div class="navheader"> + <table width="100%" summary="Navigation header"> + <tr> + <th colspan="3" align="center">Database Joins</th> + </tr> + <tr> + <td width="20%" align="left"><a accesskey="p" href="secondaryCursor.html">Prev</a> </td> + <th width="60%" align="center">Chapter 5. Secondary Databases</th> + <td width="20%" align="right"> <a accesskey="n" href="javaindexusage.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="joins"></a>Database Joins</h2> + </div> + </div> + <div></div> + </div> + <p> + If you have two or more secondary databases associated with a primary + database, then you can retrieve primary records based on the union of + multiple secondary entries. You do this using a + <span><tt class="classname">JoinCursor</tt>.</span> + + </p> + <p> + Throughout this document we have presented a + <span>class</span> + + that stores + <span>inventory</span> + information on grocery + + + That + <span>class</span> + + is fairly simple with a limited + number of data members, few of which would be interesting from a query + perspective. But suppose, instead, that we were storing + information on something with many more queryable characteristics, such + as an automobile. In that case, you may be storing information such as + color, number of doors, fuel mileage, automobile type, number of + passengers, make, model, and year, to name just a few. + </p> + <p> + In this case, you would still likely be using some unique value to key your + primary entries (in the United States, the automobile's VIN would be + ideal for this purpose). You would then create a + <span>class</span> + + that identifies + all the characteristics of the automobiles in your inventory. + + <span> + You would + also have to create some mechanism by which you would move instances of + this class in and out of Java <tt class="literal">byte</tt> arrays. We + described the concepts and mechanisms by which you can perform these + activities in <a href="DBEntry.html">Database Records</a>. + </span> + </p> + <p> + To query this data, you might then create multiple secondary databases, + one for each of the characteristics that you want to query. For + example, you might create a secondary for color, another for number of + doors, another for number of passengers, and so forth. Of course, you + will need a unique + <span>key creator</span> + + for each such secondary database. You do + all of this using the concepts and techniques described throughout this + chapter. + </p> + <p> + Once you have created this primary database and all interesting + secondaries, what you have is the ability to retrieve automobile records + based on a single characteristic. You can, for example, find all the + automobiles that are red. Or you can find all the automobiles that have + four doors. Or all the automobiles that are minivans. + </p> + <p> + The next most natural step, then, is to form compound queries, or joins. + For example, you might want to find all the automobiles that are red, + and that were built by Toyota, and that are minivans. You can do this + using a + <span><tt class="classname">JoinCursor</tt> class instance.</span> + + + </p> + <div class="sect2" lang="en" xml:lang="en"> + <div class="titlepage"> + <div> + <div> + <h3 class="title"><a id="joinUsage"></a>Using Join Cursors</h3> + </div> + </div> + <div></div> + </div> + <p> + To use a join cursor: + </p> + <div class="itemizedlist"> + <ul type="disc"> + <li> + <p> + Open two or more + <span>secondary cursors. These </span> + cursors + + <span>for</span> + secondary databases that are associated with + the same primary database. + </p> + </li> + <li> + <p> + Position each such cursor to the secondary key + value in which you are interested. For example, to build on + the previous description, the cursor for the color + database is positioned to the <tt class="literal">red</tt> records + while the cursor for the model database is positioned to the + <tt class="literal">minivan</tt> records, and the cursor for the + make database is positioned to <tt class="literal">Toyota</tt>. + </p> + </li> + <li> + <p> + Create an array of <span>secondary</span> cursors, and place in it each + of the cursors that are participating in your join query. + </p> + </li> + <li> + <p> + + Obtain a join cursor. You do this using the + <tt class="methodname">Database.join()</tt> + + + method. You must pass this method the array of secondary cursors that you + opened and positioned in the previous steps. + </p> + </li> + <li> + <p> + Iterate over the set of matching records + <span>using <tt class="methodname">JoinCursor.getNext()</tt></span> + until + <span><tt class="classname">OperationStatus</tt> is not <tt class="literal">SUCCESS</tt>.</span> + + </p> + </li> + <li> + <p> + Close your <span>join</span> cursor. + </p> + </li> + <li> + <p> + If you are done with them, close all your <span>secondary</span> cursors. + </p> + </li> + </ul> + </div> + <p> + For example: + + </p> + <a id="java_index9"></a> + <pre class="programlisting">package com.sleepycat.examples.db.GettingStarted; + +import com.sleepycat.db.Database; +import com.sleepycat.db.DatabaseEntry; +import com.sleepycat.db.DatabaseException; +import com.sleepycat.db.JoinCursor; +import com.sleepycat.db.LockMode; +import com.sleepycat.db.OperationStatus; +import com.sleepycat.db.SecondaryCursor; +import com.sleepycat.db.SecondaryDatabase; + +... + +// Database and secondary database opens omitted for brevity. +// Assume a primary database handle: +// automotiveDB +// Assume 3 secondary database handles: +// automotiveColorDB -- index based on automobile color +// automotiveTypeDB -- index based on automobile type +// automotiveMakeDB -- index based on the manufacturer +Database automotiveDB = null; +SecondaryDatabase automotiveColorDB = null; +SecondaryDatabase automotiveTypeDB = null; +SecondaryDatabase automotiveMakeDB = null; + +// Query strings: +String theColor = "red"; +String theType = "minivan"; +String theMake = "Toyota"; + +// Secondary cursors used for the query: +SecondaryCursor colorSecCursor = null; +SecondaryCursor typeSecCursor = null; +SecondaryCursor makeSecCursor = null; + +// The join cursor +JoinCursor joinCursor = null; + +// These are needed for our queries +DatabaseEntry foundKey = new DatabaseEntry(); +DatabaseEntry foundData = new DatabaseEntry(); + +// All cursor operations are enclosed in a try block to ensure that they +// get closed in the event of an exception. + +try { + // Database entries used for the query: + DatabaseEntry color = new DatabaseEntry(theColor.getBytes("UTF-8")); + DatabaseEntry type = new DatabaseEntry(theType.getBytes("UTF-8")); + DatabaseEntry make = new DatabaseEntry(theMake.getBytes("UTF-8")); + + colorSecCursor = automotiveColorDB.openSecondaryCursor(null, null); + typeSecCursor = automotiveTypeDB.openSecondaryCursor(null, null); + makeSecCursor = automotiveMakeDB.openSecondaryCursor(null, null); + + // Position all our secondary cursors to our query values. + OperationStatus colorRet = + colorSecCursor.getSearchKey(color, foundData, LockMode.DEFAULT); + OperationStatus typeRet = + typeSecCursor.getSearchKey(type, foundData, LockMode.DEFAULT); + OperationStatus makeRet = + makeSecCursor.getSearchKey(make, foundData, LockMode.DEFAULT); + + // If all our searches returned successfully, we can proceed + if (colorRet == OperationStatus.SUCCESS && + typeRet == OperationStatus.SUCCESS && + makeRet == OperationStatus.SUCCESS) { + + // Get a secondary cursor array and populate it with our + // positioned cursors + SecondaryCursor[] cursorArray = {colorSecCursor, + typeSecCursor, + makeSecCursor}; + + // Create the join cursor + joinCursor = automotiveDB.join(cursorArray, null); + + // Now iterate over the results, handling each in turn + while (joinCursor.getNext(foundKey, foundData, LockMode.DEFAULT) == + OperationStatus.SUCCESS) { + + // Do something with the key and data retrieved in + // foundKey and foundData + } + } +} catch (DatabaseException dbe) { + // Error reporting goes here +} catch (Exception e) { + // Error reporting goes here +} finally { + try { + // Make sure to close out all our cursors + if (colorSecCursor != null) { + colorSecCursor.close(); + } + if (typeSecCursor != null) { + typeSecCursor.close(); + } + if (makeSecCursor != null) { + makeSecCursor.close(); + } + if (joinCursor != null) { + joinCursor.close(); + } + } catch (DatabaseException dbe) { + // Error reporting goes here + } +} </pre> + </div> + <div class="sect2" lang="en" xml:lang="en"> + <div class="titlepage"> + <div> + <div> + <h3 class="title"><a id="joinconfig"></a>JoinCursor Properties</h3> + </div> + </div> + <div></div> + </div> + <p> + You can set <tt class="classname">JoinCursor</tt> properties using the + <tt class="classname">JoinConfig</tt> class. Currently there is just one property that you can + set: + </p> + <div class="itemizedlist"> + <ul type="disc"> + <li> + <p> + <tt class="methodname">JoinConfig.setNoSort()</tt> + </p> + <p> + Specifies whether automatic sorting of input cursors is disabled. The cursors are sorted from the + one that refers to the least number of data items to the one that refers to the most. + </p> + <p> + If the data is structured so that cursors with many data items also share many common elements, + higher performance will result from listing those cursors before cursors with fewer data + items. Turning off sorting permits applications to specify cursors in the proper order given this + scenario. + </p> + <p> + Default value is <tt class="literal">false</tt> (automatic cursor sorting is performed). + </p> + <p> + For example: + </p> + <a id="je_index10"></a> + <pre class="programlisting">// All database and environments omitted +JoinConfig config = new JoinConfig(); +config.setNoSort(true); +JoinCursor joinCursor = myDb.join(cursorArray, config); </pre> + </li> + </ul> + </div> + </div> + </div> + <div class="navfooter"> + <hr /> + <table width="100%" summary="Navigation footer"> + <tr> + <td width="40%" align="left"><a accesskey="p" href="secondaryCursor.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="javaindexusage.html">Next</a></td> + </tr> + <tr> + <td width="40%" align="left" valign="top"> + Using Secondary Cursors + + </td> + <td width="20%" align="center"> + <a accesskey="h" href="index.html">Home</a> + </td> + <td width="40%" align="right" valign="top"> Secondary Database Example</td> + </tr> + </table> + </div> + </body> +</html> |