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259 lines
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<meta name="DC.Title" content="Use UNION keyword to combine subselects" />
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<meta name="abstract" content="Using the UNION keyword, you can combine two or more subselects to form a fullselect." />
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<meta name="description" content="Using the UNION keyword, you can combine two or more subselects to form a fullselect." />
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<title>Use UNION keyword to combine subselects</title>
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<body id="rbafykeyu"><a name="rbafykeyu"><!-- --></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 UNION keyword to combine subselects</h1>
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<div><p>Using the UNION keyword, you can combine two or more subselects
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to form a fullselect.</p>
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<div class="section"><p> When SQL encounters the UNION keyword, it processes each subselect
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to form an interim result table, then it combines the interim result table
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of each subselect and deletes duplicate rows to form a combined result table.
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You can use different clauses and techniques when coding select-statements.</p>
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</div>
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<div class="section"><p>You can use UNION to eliminate duplicates when merging lists of
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values obtained from several tables. For example, you can obtain a combined
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list of employee numbers that includes:</p>
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<ul><li>People in department D11</li>
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<li>People whose assignments include projects MA2112, MA2113, and AD3111</li>
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</ul>
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</div>
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<div class="section"><p>The combined list is derived from two tables and contains no duplicates.
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To do this, specify:</p>
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<pre><strong>SELECT</strong> EMPNO
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<strong>FROM</strong> CORPDATA.EMPLOYEE
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<strong>WHERE</strong> WORKDEPT = 'D11'
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<strong>UNION</strong>
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<strong>SELECT</strong> EMPNO
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<strong>FROM</strong> CORPDATA.EMPPROJACT
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<strong>WHERE</strong> PROJNO = 'MA2112' <strong>OR</strong>
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PROJNO = 'MA2113' <strong>OR</strong>
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PROJNO = 'AD3111'
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<strong>ORDER BY</strong> EMPNO</pre>
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</div>
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<div class="section"><p>To better understand the results from these SQL statements, imagine
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that SQL goes through the following process:</p>
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</div>
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<div class="section"><p>Step 1. SQL processes the first SELECT statement: </p>
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<pre><strong>SELECT</strong> EMPNO
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<strong>FROM</strong> CORPDATA.EMPLOYEE
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<strong>WHERE</strong> WORKDEPT = 'D11'</pre>
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<p>Which results in an interim
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result table:</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="d0e105">EMPNO from CORPDATA.EMPLOYEE</th>
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</tr>
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</thead>
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<tbody><tr><td valign="top" headers="d0e105 ">000060</td>
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</tr>
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<tr><td valign="top" headers="d0e105 ">000150</td>
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</tr>
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<tr><td valign="top" headers="d0e105 ">000160</td>
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</tr>
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<tr><td valign="top" headers="d0e105 ">000170</td>
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</tr>
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<tr valign="top"><td valign="top" headers="d0e105 ">000180</td>
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</tr>
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<tr valign="top"><td valign="top" headers="d0e105 ">000190</td>
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</tr>
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<tr valign="top"><td valign="top" headers="d0e105 ">000200</td>
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</tr>
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<tr valign="top"><td valign="top" headers="d0e105 ">000210</td>
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</tr>
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<tr valign="top"><td valign="top" headers="d0e105 ">000220</td>
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</tr>
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<tr valign="top"><td valign="top" headers="d0e105 ">200170</td>
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</tr>
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<tr valign="top"><td valign="top" headers="d0e105 ">200220</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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<p>Step 2. SQL processes the second SELECT statement:</p>
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<pre><strong>SELECT</strong> EMPNO
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<strong>FROM</strong> CORPDATA.EMPPROJACT
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<strong>WHERE</strong> PROJNO='MA2112' <strong>OR</strong>
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PROJNO= 'MA2113' <strong>OR</strong>
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PROJNO= 'AD3111'</pre>
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<p>Which results in another interim
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result table:</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="d0e166">EMPNO from CORPDATA.EMPPROJACT</th>
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</tr>
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</thead>
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<tbody><tr><td valign="top" headers="d0e166 ">000230</td>
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</tr>
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<tr><td valign="top" headers="d0e166 ">000230</td>
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</tr>
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<tr><td valign="top" headers="d0e166 ">000240</td>
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</tr>
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<tr><td valign="top" headers="d0e166 ">000230</td>
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</tr>
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<tr valign="top"><td valign="top" headers="d0e166 ">000230</td>
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</tr>
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<tr valign="top"><td valign="top" headers="d0e166 ">000240</td>
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</tr>
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<tr valign="top"><td valign="top" headers="d0e166 ">000230</td>
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</tr>
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<tr valign="top"><td valign="top" headers="d0e166 ">000150</td>
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</tr>
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<tr valign="top"><td valign="top" headers="d0e166 ">000170</td>
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</tr>
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<tr valign="top"><td valign="top" headers="d0e166 ">000190</td>
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</tr>
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<tr valign="top"><td valign="top" headers="d0e166 ">000170</td>
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</tr>
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<tr valign="top"><td valign="top" headers="d0e166 ">000190</td>
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</tr>
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<tr valign="top"><td valign="top" headers="d0e166 ">000150</td>
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</tr>
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<tr valign="top"><td valign="top" headers="d0e166 ">000160</td>
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</tr>
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<tr valign="top"><td valign="top" headers="d0e166 ">000180</td>
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</tr>
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<tr valign="top"><td valign="top" headers="d0e166 ">000170</td>
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</tr>
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<tr valign="top"><td valign="top" headers="d0e166 ">000210</td>
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</tr>
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<tr valign="top"><td valign="top" headers="d0e166 ">000210</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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<p>Step 3. SQL combines the two interim result tables, removes duplicate
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rows, and orders the result:</p>
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<pre><strong>SELECT</strong> EMPNO
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<strong>FROM</strong> CORPDATA.EMPLOYEE
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<strong>WHERE</strong> WORKDEPT = 'D11'
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<strong>UNION</strong>
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<strong>SELECT</strong> EMPNO
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<strong>FROM</strong> CORPDATA.EMPPROJACT
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<strong>WHERE</strong> PROJNO='MA2112' <strong>OR</strong>
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PROJNO= 'MA2113' <strong>OR</strong>
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PROJNO= 'AD3111'
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<strong>ORDER BY</strong> EMPNO</pre>
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<p>Which results in a combined result table
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with values in ascending sequence:</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="d0e263">EMPNO</th>
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</tr>
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</thead>
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<tbody><tr><td valign="top" headers="d0e263 ">000060</td>
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</tr>
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<tr><td valign="top" headers="d0e263 ">000150</td>
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</tr>
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<tr><td valign="top" headers="d0e263 ">000160</td>
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</tr>
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<tr><td valign="top" headers="d0e263 ">000170</td>
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</tr>
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<tr valign="top"><td valign="top" headers="d0e263 ">000180</td>
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</tr>
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<tr valign="top"><td valign="top" headers="d0e263 ">000190</td>
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</tr>
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<tr valign="top"><td valign="top" headers="d0e263 ">000200</td>
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</tr>
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<tr valign="top"><td valign="top" headers="d0e263 ">000210</td>
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</tr>
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<tr valign="top"><td valign="top" headers="d0e263 ">000220</td>
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</tr>
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<tr valign="top"><td valign="top" headers="d0e263 ">000230</td>
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</tr>
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<tr valign="top"><td valign="top" headers="d0e263 ">000240</td>
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</tr>
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<tr valign="top"><td valign="top" headers="d0e263 ">200170</td>
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</tr>
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<tr valign="top"><td valign="top" headers="d0e263 ">200220</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>When you use UNION:</p>
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<ul><li>Any ORDER BY clause must appear after the last subselect that is part
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of the union. In this example, the results are sequenced on the basis of the
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first selected column, <em>EMPNO</em>. The ORDER BY clause specifies that the
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combined result table is to be in collated sequence. ORDER BY is not allowed
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in a view.</li>
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<li>A name may be specified on the ORDER BY clause if the result columns are
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named. A result column is named if the corresponding columns in each of the
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unioned select-statements have the same name. An AS clause can be used to
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assign a name to columns in the select list. <pre> <strong>SELECT</strong> A + B <strong>AS</strong> X ...
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<strong>UNION
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SELECT</strong> X ... <strong>ORDER BY</strong> X</pre>
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<div class="p">If the result columns are
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unnamed, use a positive integer to order the result. The number refers to
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the position of the expression in the list of expressions you include in your
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subselects. <pre> <strong>SELECT</strong> A + B ...
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<strong>UNION
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SELECT</strong> X ... <strong>ORDER BY</strong> 1</pre>
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</div>
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</li>
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</ul>
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</div>
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<div class="section"><p>To identify which subselect each row is from, you can include
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a constant at the end of the select list of each subselect in the union. When
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SQL returns your results, the last column contains the constant for the subselect
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that is the source of that row. For example, you can specify: </p>
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<pre> <strong>SELECT</strong> A, B, 'A1' ...
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<strong>UNION
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SELECT</strong> X, Y, 'B2'...</pre>
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</div>
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<div class="section"><p>When a row is returned, it includes a value (either A1 or B2)
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to indicate the table that is the source of the row's values. If the column
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names in the union are different, SQL uses the set of column names specified
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in the first subselect when interactive SQL displays or prints the results,
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or in the SQLDA resulting from processing an SQL DESCRIBE statement.</p>
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</div>
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<div class="section"><div class="p"> <div class="note"><span class="notetitle">Note:</span> Sort sequence is applied after the fields across the UNION
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pieces are made compatible. The sort sequence is used for the distinct processing
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that implicitly occurs during UNION processing. </div>
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</div>
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</div>
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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="rbafykeyuall.htm">Specify UNION ALL</a></strong><br />
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If you want to keep duplicates in the result of a UNION, specify UNION ALL instead of just UNION.</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="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 class="relref"><strong>Related reference</strong><br />
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<div><a href="rbafyviewnt.htm" title="A view can be used to access data in one or more tables or views. You create a view by using a SELECT statement.">Create and use views</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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