78 lines
5.1 KiB
HTML
78 lines
5.1 KiB
HTML
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<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE html
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PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html lang="en-us" xml:lang="en-us">
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
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<meta name="security" content="public" />
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<meta name="Robots" content="index,follow" />
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<meta http-equiv="PICS-Label" content='(PICS-1.1 "http://www.icra.org/ratingsv02.html" l gen true r (cz 1 lz 1 nz 1 oz 1 vz 1) "http://www.rsac.org/ratingsv01.html" l gen true r (n 0 s 0 v 0 l 0) "http://www.classify.org/safesurf/" l gen true r (SS~~000 1))' />
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<meta name="DC.Type" content="reference" />
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<meta name="DC.Title" content="Normalization" />
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<meta name="abstract" content="Several available design methods allow you to design technically correct databases, and effective relational database structure. Some of these methods are based on a design approach called normalization. Normalization refers to the reduction or elimination of storing redundant data." />
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<meta name="description" content="Several available design methods allow you to design technically correct databases, and effective relational database structure. Some of these methods are based on a design approach called normalization. Normalization refers to the reduction or elimination of storing redundant data." />
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<meta name="DC.Relation" scheme="URI" content="rzaikdbdesign.htm" />
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<meta name="copyright" content="(C) Copyright IBM Corporation 1999, 2006" />
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<meta name="DC.Rights.Owner" content="(C) Copyright IBM Corporation 1999, 2006" />
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<meta name="DC.Format" content="XHTML" />
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<meta name="DC.Identifier" content="normalization" />
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<meta name="DC.Language" content="en-us" />
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<!-- All rights reserved. Licensed Materials Property of IBM -->
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<!-- US Government Users Restricted Rights -->
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<!-- GSA ADP Schedule Contract with IBM Corp. -->
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<link rel="stylesheet" type="text/css" href="./ibmdita.css" />
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<link rel="stylesheet" type="text/css" href="./ic.css" />
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<title>Normalization</title>
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</head>
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<body id="normalization"><a name="normalization"><!-- --></a>
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<!-- Java sync-link --><script language="Javascript" src="../rzahg/synch.js" type="text/javascript"></script>
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<h1 class="topictitle1">Normalization</h1>
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<div><p>Several available design methods allow you to design technically
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correct databases, and effective relational database structure. Some of these
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methods are based on a design approach called normalization. Normalization
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refers to the reduction or elimination of storing redundant data.</p>
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<div class="section"><p> The primary objective of normalization is to avoid problems that
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are associated with updating redundant data.</p>
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</div>
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<div class="section"><p>However, this design approach of normalization (for example, 3NF–3rd
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Normal Form), may result in large numbers of tables. If there are numerous
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table join operations, SQL performance may be reduced. Consider overall SQL
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performance when you design databases. Balance<sup>®</sup> the amount of redundant data
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with the number of tables that are not fully normalized.</p>
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</div>
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<div class="section"><p>The following graphic illustrates that the proportion of redundant
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data to the number of tables affects performance:</p>
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</div>
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<div class="section"><br /><img src="perf02.gif" alt="Balancing redundant data and number of tables" /><br /></div>
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<div class="section"><p>Minimize the use of code tables when little is gained from their
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use. For example, an employee table contains a JOBCODE column, with data values
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054, 057, and so forth. This table must be joined with another table to translate
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the codes to Programmer, Engineer, and so on. The cost of this join could
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be quite high compared to the savings in storage and potential update errors
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resulting from redundant data.</p>
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<p>For example:</p>
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</div>
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<div class="section"><div class="fignone"><span class="figcap">Figure 1. Normalized data form:</span><br /><img src="perf03.gif" alt="Normalized data form" /><br /></div>
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<div class="fignone"><span class="figcap">Figure 2. Redundant data form:</span><br /><img src="perf04.gif" alt="Redundant data form" /><br /></div>
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</div>
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<div class="section"><p>The set level (or mass operation) nature of <span class="keyword">SQL</span> significantly
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lessens the danger of a certain redundant data form. For example, the ability
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to update a set of rows with a single <span class="keyword">SQL</span> statement
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greatly reduces this risk. In the following example, the job title <strong>Engineer</strong> must
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be changed to <strong>Technician</strong> for all rows that match this condition.</p>
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</div>
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<div class="example"><p>Use <span class="keyword">SQL</span> to
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update JOBTITLE:</p>
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<pre> UPDATE EMPLOYEE
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SET JOBTITLE = "Technician"
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WHERE JOBTITLE = "Engineer"</pre>
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</div>
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</div>
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<div>
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<div class="familylinks">
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<div class="parentlink"><strong>Parent topic:</strong> <a href="rzaikdbdesign.htm" title="Use the following topics to determine what tables you require in your database and to understand the relationship between those tables.">Database design</a></div>
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</div>
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</div>
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</body>
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</html>
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