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<!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" xml:lang="en" lang="en"> <script src="./callback.js" type="text/javascript"></script> <noscript>Your browser does not support JavaScript. This help page requires JavaScript to render correctly.</noscript> </head> <meta http-equiv="Content-Type" content="text/html; charset=us-ascii" /> <meta http-equiv="Content-Style-Type" content="text/css" /> <meta http-equiv="Content-Script-Type" content="text/javascript" /> <meta name="generator" content="Oracle DARB XHTML Converter (Mode = ohj/ohw) - Version 5.1.1 Build 005" /> <meta name="date" content="2009-04-21T9:46:24Z" /> <meta name="robots" content="noarchive" /> <meta name="doctitle" content="About Constraint Types" /> <meta name="relnum" content="11g Release 2 (11.2)" /> <meta name="partnum" content="E10766-01" /> <link rel="copyright" href="./dcommon/html/cpyr.htm" title="Copyright" type="text/html" /> <link rel="stylesheet" href="./dcommon/css/blafdoc.css" title="Oracle BLAFDoc" type="text/css" /> <link rel="contents" href="toc.htm" title="Contents" type="text/html" /> <link rel="prev" href="tdddg_objects008.htm" title="Previous" type="text/html" /> <link rel="next" href="tdddg_objects010.htm" title="Next" type="text/html" /> <title>About Constraint Types</title> </head> <body> <div class="zz-skip-header"><a href="#BEGIN">Skip Headers</a></div> <table class="simple oac_no_warn" summary="" cellspacing="0" cellpadding="0" width="100%"> <col width="86%" /> <col width="*" /> <tr valign="bottom"> <td align="left"></td> <td align="center"><a href="tdddg_objects008.htm"><img width="24" height="24" src="./dcommon/gifs/leftnav.gif" alt="Previous" /><br /> <span class="icon">Previous</span></a> </td> <td align="center"><a href="tdddg_objects010.htm"><img width="24" height="24" src="./dcommon/gifs/rightnav.gif" alt="Next" /><br /> <span class="icon">Next</span></a></td> </tr> </table> <p><a id="BABIBCDA" name="BABIBCDA"></a><a id="TDDDG32210" name="TDDDG32210"></a></p> <div class="sect3"> <h1>About Constraint Types</h1> <a name="BEGIN" id="BEGIN"></a> <p><a id="sthref205" name="sthref205"></a>The constraint types are:</p> <ul> <li> <p><a id="sthref206" name="sthref206"></a><span class="bold">Not Null</span>, which prevents a value from being null</p> <p>In the <code>EMPLOYEES</code> table, the column <code>LAST_NAME</code> has the <code>NOT</code> <code>NULL</code> constraint, which enforces the business rule that every employee must have a last name.</p> </li> <li> <p><a id="sthref207" name="sthref207"></a><span class="bold">Unique</span>, which prevents multiple rows from having the same value in the same column or combination of columns, but allows some values to be null</p> <p>In the <code>EMPLOYEES</code> table, the column <code>EMAIL</code> has the <code>UNIQUE</code> constraint, which enforces the business rule that an employee can have no email address, but cannot have the same email address as another employee.</p> </li> <li> <p><a id="sthref208" name="sthref208"></a><span class="bold">Primary Key</span>, which is a combination of <code>NOT</code> <code>NULL</code> and <code>UNIQUE</code></p> <p>In the <code>EMPLOYEES</code> table, the column <code>EMPLOYEE_ID</code> has the <code>PRIMARY</code> <code>KEY</code> constraint, which enforces the business rule that every employee must have a unique employee identification number.</p> </li> <li> <p><a id="sthref209" name="sthref209"></a><span class="bold">Foreign Key</span>, which requires values in one table to match values in another table</p> <p>In the <code>EMPLOYEES</code> table, the column <code>JOB_ID</code> has a <code>FOREIGN</code> <code>KEY</code> constraint that references the <code>JOBS</code> table, which enforces the business rule that an employee cannot have a <code>JOB_ID</code> that is not in the <code>JOBS</code> table.</p> </li> <li> <p><a id="sthref210" name="sthref210"></a><span class="bold">Check</span>, which requires that a value satisfy a specified condition</p> <p>The <code>EMPLOYEES</code> table does not have <code>CHECK</code> constraints. However, suppose that <code>EMPLOYEES</code> needs a new column, <code>EMPLOYEE_AGE</code>, and that every employee must be at least 18. The constraint <code>CHECK</code> <code>(EMPLOYEE_AGE</code> <code>>=</code> <code>18)</code> enforces the business rule.</p> <div align="center"> <div class="inftblnotealso"><br /> <table class="NoteAlso oac_no_warn" summary="" cellpadding="3" cellspacing="0"> <tbody> <tr> <td align="left"> <p class="notep1">Tip:</p> Use check constraints only when other constraint types cannot provide the necessary checking.</td> </tr> </tbody> </table> <br /></div> <!-- class="inftblnotealso" --></div> </li> <li> <p><a id="sthref211" name="sthref211"></a><span class="bold">REF</span>, which further describes the relationship between the column and the object that it references</p> <p>For information about REF constraints, see <a href="javascript:open('http://www.oracle.com/pls/db112/lookup?id=CNCPT1649','newWindow').focus()"><span class="italic">Oracle Database Concepts</span></a>.</p> </li> </ul> <div class="helpinfonotealso"> <h2>Related Topics</h2> <p><a href="javascript:open('http://www.oracle.com/pls/db112/lookup?id=CNCPT122','newWindow').focus()"><span class="italic">Oracle Database Concepts</span></a></p> <p><a href="javascript:open('http://www.oracle.com/pls/db112/lookup?id=SQLRF01111','newWindow').focus()"><span class="italic">Oracle Database SQL Language Reference</span></a></p> <p><a href="tdddg_objects008.htm#BABJFCCE">Ensuring Data Integrity in Tables</a></p> </div> </div> <!-- class="sect3" --> <!-- Start Footer --> <div class="footer"> <table class="simple oac_no_warn" summary="" cellspacing="0" cellpadding="0" width="100%"> <col width="86%" /> <col width="*" /> <tr> <td align="left"><span class="copyrightlogo">Copyright © 1996, 2009, Oracle and/or its affiliates. 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