90 lines
5.6 KiB
HTML
90 lines
5.6 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="Handle duplicate rows" />
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<meta name="abstract" content="When SQL evaluates a select-statement, several rows might qualify to be in the result table, depending on the number of rows that satisfy the select-statement's search condition. Some of the rows in the result table might be duplicates." />
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<meta name="DC.subject" content="SELECT statement, preventing duplicate rows, statements, SELECT, row, preventing duplicate, examples, preventing duplicate rows, DISTINCT keyword, keyword, DISTINCT" />
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<title>Handle duplicate rows</title>
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</head>
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<body id="rbafydupl"><a name="rbafydupl"><!-- --></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">Handle duplicate rows</h1>
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<div><p>When SQL evaluates a select-statement, several rows might qualify
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to be in the result table, depending on the number of rows that satisfy the
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select-statement's search condition. Some of the rows in the result table
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might be duplicates.</p>
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<div class="section"><p>You can specify that you do not want any duplicates by using the
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DISTINCT keyword, followed by the list of expressions: </p>
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<pre><strong>SELECT DISTINCT</strong> JOB, SEX
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…</pre>
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</div>
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<div class="section"><p>DISTINCT means you want to select only the unique rows. If a selected
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row duplicates another row in the result table, the duplicate row is ignored
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(it is not put into the result table). For example, suppose you want a list
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of employee job codes. You do not need to know which employee has what job
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code. Because it is probable that several people in a department have the
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same job code, you can use DISTINCT to ensure that the result table has only
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unique values.</p>
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</div>
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<div class="section"><p>The following example shows how to do this:</p>
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<pre> <strong>SELECT DISTINCT</strong> JOB
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<strong>FROM</strong> CORPDATA.EMPLOYEE
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<strong>WHERE</strong> WORKDEPT = 'D11'</pre>
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</div>
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<div class="section"><p>The result is two rows:</p>
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<div class="tablenoborder"><table cellpadding="4" cellspacing="0" summary="" frame="hsides" border="1" rules="all"><thead align="left"><tr valign="top"><th valign="top" id="d0e74">JOB</th>
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</tr>
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</thead>
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<tbody><tr><td valign="top" headers="d0e74 ">DESIGNER</td>
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</tr>
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<tr><td valign="top" headers="d0e74 ">MANAGER</td>
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</tr>
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</tbody>
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</table>
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</div>
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</div>
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<div class="section"><p>If you do not include DISTINCT in a SELECT clause, you might find
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duplicate rows in your result, because SQL returns the <em>JOB</em> column's
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value for each row that satisfies the search condition. Null values are treated
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as duplicate rows for DISTINCT.</p>
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</div>
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<div class="section"><p>If you include DISTINCT in a SELECT clause and you also include
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a shared-weight sort sequence, fewer values might be returned. The sort sequence
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causes values that contain the same characters to be weighted the same. If
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'MGR', 'Mgr', and 'mgr' were all in the same table, only one of these values
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is returned. </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="rbafytexas.htm" title="Learn a variety of ways of tailoring your query to gather data using the SELECT statement. One way to do this is to use the SELECT statement in a program to retrieve a specific row (for example, the row for an employee). Furthermore, you can use clauses to gather data in a specific way.">Retrieve data using the SELECT statement</a></div>
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</div>
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<div class="relconcepts"><strong>Related concepts</strong><br />
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<div><a href="rbafyussisql.htm" title="A sort sequence defines how characters in a character set relate to each other when they are compared or ordered. Normalization allows you to compare strings that contain combining characters.">Sort sequences and normalization in SQL</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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