66 lines
4.2 KiB
HTML
66 lines
4.2 KiB
HTML
<?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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<meta name="DC.Type" content="concept" />
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<meta name="DC.Title" content="Use subqueries" />
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<meta name="abstract" content="You can use subqueries in a search condition as another way to select your data. Subqueries can be used anywhere an expression can be used." />
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<meta name="description" content="You can use subqueries in a search condition as another way to select your data. Subqueries can be used anywhere an expression can be used." />
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<meta name="DC.Relation" scheme="URI" content="rbafywhereexp.htm" />
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<meta name="copyright" content="(C) Copyright IBM Corporation 1998, 2006" />
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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>Use subqueries</title>
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</head>
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<body id="rbafysubquery"><a name="rbafysubquery"><!-- --></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">Use subqueries</h1>
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<div><p>You can use subqueries in a search condition as another
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way to select your data. Subqueries can be used anywhere an expression can
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be used.</p>
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<p>Conceptually, a subquery is evaluated whenever a new row or group of rows
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must be processed. In fact, if the subquery is the same for every row or group,
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it is evaluated only once. Subqueries like this are said to be <strong>uncorrelated</strong>.</p>
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<p>Some subqueries return different values from row to row or group to group.
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The mechanism that allows this is called <strong>correlation</strong>, and the subqueries
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are said to be <strong>correlated</strong>.</p>
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</div>
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<div>
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<ul class="ullinks">
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<li class="ulchildlink"><strong><a href="rbafysubq.htm">Subqueries in SELECT statements</a></strong><br />
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Subqueries can help you to further refine your search conditions.</li>
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<li class="ulchildlink"><strong><a href="rbafycorrs.htm">Correlated subqueries</a></strong><br />
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You can write a subquery that SQL might need to re-evaluate as
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it examines each new row (WHERE clause) or group of rows (HAVING clause) in
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the outer-level SELECT. This is called a <dfn class="term">correlated subquery</dfn>.</li>
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</ul>
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<div class="familylinks">
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<div class="parentlink"><strong>Parent topic:</strong> <a href="rbafydml.htm" title="Data manipulation language (DML) describes the portion of SQL that allows you to manipulate or control your data.">Data manipulation language</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="rbafywhereexp.htm" title="An expression in a WHERE clause names or specifies something you want to compare to something else.">Expressions in the WHERE clause</a></div>
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<div><a href="rbafycomsrch.htm" title="In addition to the basic comparison predicates (=, >, <, and so on), a search condition can contain any of the predicates BETWEEN, IN, EXISTS, IS NULL, and LIKE.">Define complex search conditions</a></div>
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</div>
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</div>
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</body>
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</html> |