129 lines
6.8 KiB
HTML
129 lines
6.8 KiB
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<meta name="abstract" content="Subqueries can help you to further refine your search conditions." />
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<title>Subqueries in SELECT statements</title>
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<body id="rbafysubq"><a name="rbafysubq"><!-- --></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">Subqueries in SELECT statements</h1>
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<div><p>Subqueries can help you to further refine your search conditions.</p>
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<div class="section"><p>In simple WHERE and HAVING clauses, you can specify a search condition
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by using a literal value, a column name, an expression, or a special register.
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In those search conditions, you know that you are searching for a specific
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value. However, sometimes you cannot supply that value until you have retrieved
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other data from a table. For example, suppose you want a list of the employee
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numbers, names, and job codes of all employees working on a particular project,
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say project number MA2100. The first part of the statement is easy to write: </p>
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<pre> <strong>SELECT</strong> EMPNO, LASTNAME, JOB
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<strong>FROM</strong> CORPDATA.EMPLOYEE
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<strong>WHERE</strong> EMPNO …</pre>
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</div>
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<div class="section"><p>But you cannot go further because the CORPDATA.EMPLOYEE table
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does not include project number data. You do not know which employees are
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working on project MA2100 without issuing another SELECT statement against
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the CORPDATA.EMP_ACT table.</p>
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</div>
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<div class="section"><p>With SQL, you can nest one SELECT statement within another to
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solve this problem. The inner SELECT statement is called a <strong>subquery</strong>.
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The SELECT statement surrounding the subquery is called the <strong>outer-level
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SELECT</strong>. Using a subquery, you can issue just one SQL statement to retrieve
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the employee numbers, names, and job codes for employees who work on project
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MA2100: </p>
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<pre> <strong>SELECT</strong> EMPNO, LASTNAME, JOB
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<strong>FROM</strong> CORPDATA.EMPLOYEE
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<strong>WHERE</strong> EMPNO <strong>IN</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 = 'MA2100')</pre>
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</div>
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<div class="section"><p>To better understand what will result from this SQL statement,
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imagine that SQL goes through the following process:</p>
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</div>
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<div class="section"><p>Step 1: SQL evaluates the subquery to obtain a list of EMPNO values: </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= 'MA2100')</pre>
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<p>Which results
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in an interim results 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="d0e112">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="d0e112 ">000010</td>
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</tr>
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<tr><td valign="top" headers="d0e112 ">000110</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: The interim results table then serves as a list in the
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search condition of the outer-level SELECT. Essentially, this is the statement
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that is run.</p>
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<pre><strong>SELECT</strong> EMPNO, LASTNAME, JOB
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<strong>FROM</strong> CORPDATA.EMPLOYEE
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<strong>WHERE</strong> EMPNO <strong>IN</strong>
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('000010', '000110')</pre>
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<p>The final result table looks
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like this:</p>
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<div class="tablenoborder"><table cellpadding="4" cellspacing="0" summary="" frame="hsides" border="1" rules="all"><thead align="left"><tr><th valign="top" id="d0e145">EMPNO</th>
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<th valign="top" id="d0e147">LASTNAME</th>
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<th valign="top" id="d0e149">JOB</th>
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</tr>
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</thead>
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<tbody><tr><td valign="top" headers="d0e145 ">000010</td>
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<td valign="top" headers="d0e147 ">HAAS</td>
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<td valign="top" headers="d0e149 ">PRES</td>
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</tr>
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<tr><td valign="top" headers="d0e145 ">000110</td>
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<td valign="top" headers="d0e147 ">LUCCHESSI</td>
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<td valign="top" headers="d0e149 ">SALESREP</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>
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<div>
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<ul class="ullinks">
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<li class="ulchildlink"><strong><a href="rbafysubnsearch.htm">Subqueries and search conditions</a></strong><br />
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A subquery can be part of a search condition.</li>
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<li class="ulchildlink"><strong><a href="rbafysubquerynotes.htm">Usage notes on subqueries</a></strong><br />
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When using subqueries, you should be aware of these usage notes.</li>
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<li class="ulchildlink"><strong><a href="rbafyhowsub.htm">Include subqueries in WHERE or HAVING clauses</a></strong><br />
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Here are several ways you can use to include a subquery in either a WHERE or HAVING clause.</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="rbafysubquery.htm" title="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.">Use subqueries</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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