90 lines
5.6 KiB
HTML
90 lines
5.6 KiB
HTML
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<title>Example: Correlated subquery in a HAVING clause</title>
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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">Example: Correlated subquery in a HAVING clause</h1>
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<div><p>Suppose that you want a list of all the departments whose average
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salary is higher than the average salary of their area (all departments whose
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WORKDEPT begins with the same letter belong to the same area). To get this
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information, SQL must search the CORPDATA.EMPLOYEE table.</p>
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<div class="section"><p>For each department in the table, SQL compares the department's
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average salary to the average salary of the area. In the subquery, SQL calculates
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the average salary for the area of the department in the current group. For
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example:</p>
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<pre> <strong>SELECT</strong> WORKDEPT, <strong>DECIMAL</strong>(<strong>AVG</strong>(SALARY),8,2)
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<strong>FROM</strong> CORPDATA.EMPLOYEE X
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<strong>GROUP BY</strong> WORKDEPT
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<strong>HAVING AVG</strong>(SALARY) >
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(<strong>SELECT AVG</strong>(SALARY)
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<strong>FROM</strong> CORPDATA.EMPLOYEE
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<strong>WHERE SUBSTR</strong>(X.WORKDEPT,1,1) = <strong>SUBSTR</strong>(WORKDEPT,1,1))</pre>
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</div>
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<div class="section"><p>Consider what happens when the subquery is executed for a given
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department of CORPDATA.EMPLOYEE. Before it is executed, the occurrence of
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X.WORKDEPT is replaced with the value of the WORKDEPT column for that group.
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Suppose, for example, that the first group selected has A00 for the value
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of WORKDEPT. The subquery executed for this group is: </p>
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<pre> (<strong>SELECT AVG</strong>(SALARY)
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<strong>FROM</strong> CORPDATA.EMPLOYEE
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<strong>WHERE SUBSTR</strong>('A00',1,1) = <strong>SUBSTR</strong>(WORKDEPT,1,1))</pre>
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</div>
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<div class="section"><p>Thus, for the group considered, the subquery produces the average
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salary for the area. This value is then compared in the outer statement to
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the average salary for department 'A00'. For some other group for which WORKDEPT
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is 'B01', the subquery results in the average salary for the area where department
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B01 belongs.</p>
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</div>
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<div class="section"><p>The result table produced by the query has the following values:</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="d0e95">WORKDEPT</th>
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<th valign="top" id="d0e97">AVG SALARY</th>
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</tr>
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</thead>
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<tbody><tr><td valign="top" headers="d0e95 ">D21</td>
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<td valign="top" headers="d0e97 ">25668.57</td>
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</tr>
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<tr><td valign="top" headers="d0e95 ">E01</td>
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<td valign="top" headers="d0e97 ">40175.00</td>
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</tr>
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<tr><td valign="top" headers="d0e95 ">E21</td>
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<td valign="top" headers="d0e97 ">24086.66</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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<div class="familylinks">
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<div class="parentlink"><strong>Parent topic:</strong> <a href="rbafycorrs.htm" title="You can write a subquery that SQL might need to re-evaluate as it examines each new row (WHERE clause) or group of rows (HAVING clause) in the outer-level SELECT. This is called a correlated subquery.">Correlated subqueries</a></div>
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</div>
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</div>
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</body>
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