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<html><head><title>Databases and Perl (Perl in a Nutshell, 2nd Edition)</title><link rel="stylesheet" type="text/css" href="../style/style1.css" /><meta name="DC.Creator" content="Stephen Spainhour" /><meta name="DC.Format" content="text/xml" scheme="MIME" /><meta name="DC.Language" content="en-US" /><meta name="DC.Publisher" content="O'Reilly & Associates, Inc." /><meta name="DC.Source" scheme="ISBN" content="0596002416L" /><meta name="DC.Subject.Keyword" content="stuff" /><meta name="DC.Title" content="Perl in a Nutshell, 2nd Edition" /><meta name="DC.Type" content="Text.Monograph" /></head><body bgcolor="#ffffff"><img src="gifs/smbanner.gif" usemap="#banner-map" border="0" alt="Book Home" /><map name="banner-map"><area shape="rect" coords="1,-2,616,66" href="index.htm" alt="Java and XSLT" /><area shape="rect" coords="629,-11,726,25" href="jobjects/fsearch.htm" alt="Search this book" /></map><div class="navbar"><table width="684" border="0"><tr><td align="left" valign="top" width="228"><a href="part5.htm"><img src="../gifs/txtpreva.gif" alt="Previous" border="0" /></a></td><td align="center" valign="top" width="228" /><td align="right" valign="top" width="228"><a href="ch12_02.htm"><img src="../gifs/txtnexta.gif" alt="Next" border="0" /></a></td></tr></table></div><h1 class="chapter">Chapter 12. Databases and Perl</h1><div class="htmltoc"><h4 class="tochead">Contents:</h4> <p> <a href="#perlnut2-CHP-12-SECT-1">DBM Databases and DBM Hashes</a><br /><a href="ch12_02.htm">Design of DBI</a><br /><a href="ch12_03.htm">DBI Methods</a><br /><a href="ch12_04.htm">DBI Environment Variables</a><br /></p></div><p>Since one of Perl's greatest strengths is workingwith text, a genuine concern is how to store data. Flat files are onepossibility, but they don't scale very well forlarge amounts of data. When working with lots of data,you'll likely need database software to satisfy yourcapacity and performance requirements.</p><p><a name="INDEX-1742" /></a>Thereare two general solutions to using databases with Perl. For simpledatabase purposes, Database Management (DBM) will serve your needs.DBM is a library supported by many (if not all) Unix systems and manynon-Unix systems as well. DBM is more efficient than flat text filesbecause of how it packs records into the database files and the(large) size of data it can store and retrieve efficiently.Perl's interface to your system'sDBM is based on a hash, so you can add, store, and delete data bykey.</p><p>When you need to store a larger variety of data and need othergoodies such as searchability on multiple records,you'll probably need to use a database that supportsSQL. To this end, you can buy a prepackaged database product such asOracle or Sybase, or a shareware equivalent such as MySQL orPostgreSQL. For these larger database projects, you should use<a name="INDEX-1743" /></a><a name="INDEX-1744" /></a>DBI and DBD. DBI is a module thatprovides a consistent interface for database solutions. DBD is adatabase-specific driver that translates DBI calls as needed for thatdatabase.</p><p>In this chapter, we'll cover DBM and talk at lengthabout DBI/DBD.</p><div class="sect1"><a name="perlnut2-CHP-12-SECT-1" /></a><h2 class="sect1">12.1. DBM Databases and DBM Hashes</h2><p><a name="INDEX-1745" /></a><a name="INDEX-1746" /></a>DBM is a simpledatabase management facility for Unix systems. It allows programs tostore a collection of key/value pairs in binary form, thus providingrudimentary database support for Perl. Practically all Unix systemsship with built-in DBM support, some with a separate libdb and otherswith DBM calls built into libc. In the absence of DBM support on yoursystem, you can use gdbm from GNU, which is an extension to vanillaDBM or BerkeleyDB-3.x from <a href="http://www.sleepycat.com/">http://www.sleepycat.com/</a>.</p><p>To use DBM databases in Perl, you can associate a hash with a DBMdatabase through the AnyDBM module that uses <tt class="literal">tie()</tt>. This hash (called a DBM array) is then used to accessand modify the DBM database.<a name="INDEX-1747" /></a>Previously, youcould use <tt class="literal">dbmopen( )</tt> to open, read, write, anddelete a database, but while <tt class="literal">dbmopen( )</tt> remainsavailable, you should use the AnyDBM module that'salways suited to your underlying DBM implementation.<a href="#FOOTNOTE-6">[6]</a> </p><blockquote class="footnote"><a name="FOOTNOTE-6" /></a><p> [6]If you're using BerkeleyDB-2.x or newer, youshould not use AnyDBM_File, but should instead install and use theBerkeleyDB module.</p> </blockquote><p>For example, with AnyDBM:</p><blockquote><pre class="code">#!/usr/local/bin/perl -w use AnyDBM_File; use Fcntl; # needed for O_ thingies my %h; my $db_name = 'perl_in_a_nutshell2.dbmx'; # tie %h. will fail if $db_name can't be created and $db_name can't be # written tie(%h, 'AnyDBM_File', $db_name, O_RDWR|O_CREAT, 0640) or die("can't create \%h: $!"); # Populate %h foreach my $letter ('a' .. 'z') { $h{$letter} = uc($letter); } while(my($key, $value) = each(%h)) { print "$key -> $value\n"; } untie(%h);</pre></blockquote><p>The <tt class="literal">%ARRAYNAME</tt> parameter is a Perl hash. (If italready has values, the values are discarded under DBM_File modules,although you can have multiple keys under BerkeleyDB-2.x and newer.)<tt class="literal">%ARRAYNAME</tt> is tied to a DBM database called<tt class="literal">$db_name</tt>. This database may be stored on disk as asingle file, or as two files called <em class="emphasis">$db_name.dir</em>and <em class="emphasis">$db_name.pag</em>, depending on the DBMimplementation.</p><p>The <tt class="literal">$mode</tt> parameter is a number that controls thepermissions of the pair of files if the files need to be created. Thenumber is in octal, so make sure that you use permissions such as<tt class="literal">0640</tt> instead of <tt class="literal">640</tt>, which aredifferent numbers in octal. If the files already exist,<tt class="literal">$mode</tt> has no effect. For example:</p><blockquote><pre class="code">tie(%BOOKS, "bookdb", O_RDWR|O_CREAT, 0666); # Open %BOOKS onto bookdb</pre></blockquote><p><a name="INDEX-1748" /></a>This invocation associates the hash<tt class="literal">%BOOKS</tt> with the disk files<em class="emphasis">bookdb.dir</em> and <em class="emphasis">bookdb.pag</em>in the current directory. If the files don't alreadyexist, they are created with a mode of <tt class="literal">0666</tt>,modified by the current <em class="emphasis">umask</em>.</p><p><tt class="literal">tie( )</tt> returns <tt class="literal">undef</tt> upon anyfailure in opening the <tt class="literal">$db_name</tt> and sets<tt class="literal">$!</tt>.</p><p>Once the database is opened, anything you do to the DBM hash isimmediately written to the database. See <a href="ch04_01.htm">Chapter 4, "The Perl Language"</a>for more information on hashes.</p><blockquote><pre class="code">tie(%BOOKS, "bookdb", O_RDWR|O_CREAT, 0666) or die("can't open bookdb: $!"); # Open %BOOKS onto bookdb $BOOKS{"1-56592-286-7"} = "Perl in a Nutshell";</pre></blockquote><p>The DBM array stays open throughout the program. When the programtermi- nates, the association is terminated. You can also break theassociation in a manner similar to closing a filehandle by using<tt class="literal">untie( )</tt>. See <a href="ch05_01.htm">Chapter 5, "Function Reference"</a> formore information on <tt class="literal">tie</tt>. <a name="INDEX-1749" /></a><a name="INDEX-1750" /></a></p></div><hr width="684" align="left" /><div class="navbar"><table width="684" border="0"><tr><td align="left" valign="top" width="228"><a href="part5.htm"><img src="../gifs/txtpreva.gif" alt="Previous" border="0" /></a></td><td align="center" valign="top" width="228"><a href="index.htm"><img src="../gifs/txthome.gif" alt="Home" border="0" /></a></td><td align="right" valign="top" width="228"><a href="ch12_02.htm"><img src="../gifs/txtnexta.gif" alt="Next" border="0" /></a></td></tr><tr><td align="left" valign="top" width="228">V. Databases</td><td align="center" valign="top" width="228"><a href="index/index.htm"><img src="../gifs/index.gif" alt="Book Index" border="0" /></a></td><td align="right" valign="top" width="228">12.2. Design of DBI</td></tr></table></div><hr width="684" align="left" /><img src="../gifs/navbar.gif" usemap="#library-map" border="0" alt="Library Navigation Links" /><p><p><font size="-1"><a href="copyrght.htm">Copyright © 2002</a> O'Reilly & Associates. All rights reserved.</font></p><map name="library-map"><area shape="rect" coords="1,0,85,94" href="../index.htm"><area shape="rect" coords="86,1,178,103" href="../lwp/index.htm"><area shape="rect" coords="180,0,265,103" href="../lperl/index.htm"><area shape="rect" coords="267,0,353,105" href="../perlnut/index.htm"><area shape="rect" coords="354,1,446,115" href="../prog/index.htm"><area shape="rect" coords="448,0,526,132" href="../tk/index.htm"><area shape="rect" coords="528,1,615,119" href="../cookbook/index.htm"><area shape="rect" coords="617,0,690,135" href="../pxml/index.htm"> </map></body></html>⌨️ 快捷键说明
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