77 lines
4.9 KiB
HTML
77 lines
4.9 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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<meta name="DC.Title" content="Comparison of identity columns and sequences" />
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<meta name="abstract" content="While IDENTITY columns and sequences are similar in many ways, there are also differences." />
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<meta name="description" content="While IDENTITY columns and sequences are similar in many ways, there are also differences." />
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<meta name="DC.subject" content="sequences, compare with identity, identity column, compare with sequence" />
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<meta name="keywords" content="sequences, compare with identity, identity column, compare with sequence" />
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<meta name="DC.Relation" scheme="URI" content="rbafysequence.htm" />
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<meta name="DC.Relation" scheme="URI" content="rbafysqlpidentity.htm" />
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<meta name="copyright" content="(C) Copyright IBM Corporation 1998, 2006" />
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<title>Comparison of identity columns and sequences</title>
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</head>
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<body id="rbafyidentsequence"><a name="rbafyidentsequence"><!-- --></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">Comparison of identity columns and sequences</h1>
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<div><p>While IDENTITY columns and sequences are similar in many ways,
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there are also differences. </p>
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<div class="section"><p>Examine these differences before you decide which to use.</p>
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</div>
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<div class="section"><p>An identity column has the following characteristics: </p>
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<ul><li>An identity column can be defined as part of a table only when the table
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is created. Once a table is created, you cannot alter it to add an identity
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column. (However, existing identity column characteristics may be altered.)</li>
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<li>An identity column automatically generates values for a single table.</li>
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<li>When an identity column is defined as GENERATED ALWAYS, the values used
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are always generated by the database manager. Applications are not allowed
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to provide their own values during the modification of the contents of the
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table.</li>
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<li>The IDENTITY_VAL_LOCAL function can be used to see the most recently assigned
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value for an identity column.</li>
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</ul>
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<p>A sequence has the following characteristics:</p>
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<ul><li> A sequence is a system object of type *DTAARA that is not tied to a table.</li>
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<li>A sequence generates sequential values that can be used in any SQL statement.</li>
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<li>There are two expressions used to retrieve the next values in the sequence
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and to look at the previous value assigned for the sequence. The PREVIOUS
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VALUE expression returns the most recently generated value for the specified
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sequence for a previous statement within the current session. The NEXT VALUE
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expression returns the next value for the specified sequence. The use of these
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expressions allows the same value to be used across several SQL statements
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within several tables.</li>
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</ul>
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<p>While these are not all of the characteristics of these two items,
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these characteristics will assist you in determining which to use depending
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on your database design and the applications using the database.</p>
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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="rbafysequence.htm" title="A sequence is an object that allows you to generate values quickly and easily.">Create and use sequences</a></div>
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</div>
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<div class="relref"><strong>Related reference</strong><br />
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<div><a href="rbafysqlpidentity.htm" title="Every time that a new row is added to a table with an identity column, the identity column value in the new row is incremented (or decremented) by the system.">Create and alter an identity column</a></div>
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<div><a href="rbafysequence.htm" title="A sequence is an object that allows you to generate values quickly and easily.">Create and use sequences</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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