238 lines
12 KiB
HTML
238 lines
12 KiB
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<meta name="DC.Title" content="GROUP BY clause" />
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<meta name="abstract" content="The GROUP BY clause allows you to find the characteristics of groups of rows rather than individual rows." />
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<meta name="description" content="The GROUP BY clause allows you to find the characteristics of groups of rows rather than individual rows." />
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<title>GROUP BY clause</title>
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<body id="rbafygroup"><a name="rbafygroup"><!-- --></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">GROUP BY clause</h1>
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<div><p>The GROUP BY clause allows you to find the characteristics of groups
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of rows rather than individual rows.</p>
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<div class="section"><p>When you specify a GROUP BY clause, SQL divides the selected rows
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into groups such that the rows of each group have matching values in one or
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more columns or expressions. Next, SQL processes each group to produce a single-row
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result for the group. You can specify one or more columns or expressions
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in the GROUP BY clause to group the rows. The items you specify in the SELECT
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statement are properties of each group of rows, not properties of individual
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rows in a table or view.</p>
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<p>Without a GROUP BY clause, the application
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of SQL aggregate functions returns <em>one</em> row. When GROUP BY is used,
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the function is applied to <em>each</em> group, thereby returning
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as many rows as there are groups.</p>
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</div>
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<div class="section"><p>For example, the CORPDATA.EMPLOYEE table has several sets of rows,
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and each set consists of rows describing members of a specific department.
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To find the average salary of people in each department, you can issue:</p>
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</div>
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<div class="section"><pre><strong>SELECT</strong> WORKDEPT, <strong>DECIMAL</strong> (<strong>AVG</strong>(SALARY),5,0)
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<strong>FROM</strong> CORPDATA.EMPLOYEE
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<strong>GROUP BY</strong> WORKDEPT</pre>
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</div>
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<div class="section"><p>The result is several rows, one for each department.</p>
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</div>
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<div class="section">
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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="d0e87">WORKDEPT</th>
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<th valign="top" id="d0e89">AVG-SALARY</th>
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</tr>
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</thead>
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<tbody><tr><td valign="top" headers="d0e87 ">A00</td>
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<td valign="top" headers="d0e89 ">40850</td>
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</tr>
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<tr><td valign="top" headers="d0e87 ">B01</td>
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<td valign="top" headers="d0e89 ">41250</td>
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</tr>
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<tr><td valign="top" headers="d0e87 ">C01</td>
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<td valign="top" headers="d0e89 ">29722</td>
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</tr>
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<tr><td valign="top" headers="d0e87 ">D11</td>
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<td valign="top" headers="d0e89 ">25147</td>
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</tr>
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<tr><td valign="top" headers="d0e87 ">D21</td>
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<td valign="top" headers="d0e89 ">25668</td>
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</tr>
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<tr><td valign="top" headers="d0e87 ">E01</td>
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<td valign="top" headers="d0e89 ">40175</td>
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</tr>
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<tr><td valign="top" headers="d0e87 ">E11</td>
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<td valign="top" headers="d0e89 ">21020</td>
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</tr>
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<tr><td valign="top" headers="d0e87 ">E21</td>
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<td valign="top" headers="d0e89 ">24086</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 class="note"><span class="notetitle">Notes:</span> <ol><li>Grouping the rows does not mean ordering them. Grouping puts each selected
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row in a group, which SQL then processes to derive characteristics of the
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group. Ordering the rows puts all the rows in the results table in ascending
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or descending collating sequence. Depending
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on the implementation selected by the database manager, the resulting groups
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may appear to be ordered.</li>
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<li>If there are null values in the column you specify in the GROUP BY clause,
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a single-row result is produced for the data in the rows with null values.</li>
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<li>If the grouping occurs over character, or UCS-2 or UTF-16 graphic columns,
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the sort sequence in effect when the query is run is applied to the grouping. </li>
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</ol>
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</div>
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</div>
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<div class="section"><p>When you use GROUP BY, you list the columns or expressions you
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want SQL to use to group the rows. For example, suppose you want a list of
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the number of people working on each major project described in the CORPDATA.PROJECT
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table. You can issue:</p>
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</div>
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<div class="section"><pre><strong>SELECT</strong> <strong>SUM</strong>(PRSTAFF), MAJPROJ
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<strong> FROM</strong> CORPDATA.PROJECT
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<strong> GROUP BY</strong> MAJPROJ</pre>
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</div>
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<div class="section"><p>The result is a list of the company's current major projects and
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the number of people working on each project: </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="d0e167">SUM(PRSTAFF)</th>
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<th valign="top" id="d0e169">MAJPROJ</th>
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</tr>
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</thead>
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<tbody><tr><td valign="top" headers="d0e167 ">6</td>
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<td valign="top" headers="d0e169 ">AD3100</td>
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</tr>
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<tr><td valign="top" headers="d0e167 ">5</td>
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<td valign="top" headers="d0e169 ">AD3110</td>
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</tr>
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<tr><td valign="top" headers="d0e167 ">10</td>
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<td valign="top" headers="d0e169 ">MA2100</td>
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</tr>
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<tr><td valign="top" headers="d0e167 ">8</td>
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<td valign="top" headers="d0e169 ">MA2110</td>
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</tr>
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<tr><td valign="top" headers="d0e167 ">5</td>
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<td valign="top" headers="d0e169 ">OP1000</td>
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</tr>
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<tr><td valign="top" headers="d0e167 ">4</td>
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<td valign="top" headers="d0e169 ">OP2000</td>
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</tr>
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<tr><td valign="top" headers="d0e167 ">3</td>
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<td valign="top" headers="d0e169 ">OP2010</td>
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</tr>
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<tr><td valign="top" headers="d0e167 ">32.5</td>
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<td valign="top" headers="d0e169 ">?</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>You can also specify that you want the rows grouped by more than
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one column or expression. For example, you can issue a select-statement to
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find the average salary for men and women in each department, using the CORPDATA.EMPLOYEE
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table. To do this, you can issue:</p>
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</div>
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<div class="section"><pre><strong>SELECT</strong> WORKDEPT, SEX, <strong>DECIMAL</strong>(<strong>AVG</strong>(SALARY),5,0) <strong>AS</strong> AVG_WAGES
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<strong>FROM</strong> CORPDATA.EMPLOYEE
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<strong>GROUP BY</strong> WORKDEPT, SEX</pre>
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</div>
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<div class="section"><p>Results in: </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" width="33.33333333333333%" id="d0e245">WORKDEPT</th>
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<th valign="top" width="33.33333333333333%" id="d0e247">SEX</th>
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<th valign="top" width="33.33333333333333%" id="d0e249">AVG_WAGES</th>
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</tr>
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</thead>
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<tbody><tr><td valign="top" width="33.33333333333333%" headers="d0e245 ">A00</td>
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<td valign="top" width="33.33333333333333%" headers="d0e247 ">F</td>
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<td valign="top" width="33.33333333333333%" headers="d0e249 ">49625</td>
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</tr>
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<tr><td valign="top" width="33.33333333333333%" headers="d0e245 ">A00</td>
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<td valign="top" width="33.33333333333333%" headers="d0e247 ">M</td>
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<td valign="top" width="33.33333333333333%" headers="d0e249 ">35000</td>
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</tr>
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<tr><td valign="top" width="33.33333333333333%" headers="d0e245 ">B01</td>
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<td valign="top" width="33.33333333333333%" headers="d0e247 ">M</td>
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<td valign="top" width="33.33333333333333%" headers="d0e249 ">41250</td>
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</tr>
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<tr><td valign="top" width="33.33333333333333%" headers="d0e245 ">C01</td>
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<td valign="top" width="33.33333333333333%" headers="d0e247 ">F</td>
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<td valign="top" width="33.33333333333333%" headers="d0e249 ">29722</td>
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</tr>
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<tr><td valign="top" width="33.33333333333333%" headers="d0e245 ">D11</td>
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<td valign="top" width="33.33333333333333%" headers="d0e247 ">F</td>
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<td valign="top" width="33.33333333333333%" headers="d0e249 ">25817</td>
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</tr>
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<tr><td valign="top" width="33.33333333333333%" headers="d0e245 ">D11</td>
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<td valign="top" width="33.33333333333333%" headers="d0e247 ">M</td>
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<td valign="top" width="33.33333333333333%" headers="d0e249 ">24764</td>
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</tr>
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<tr><td valign="top" width="33.33333333333333%" headers="d0e245 ">D21</td>
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<td valign="top" width="33.33333333333333%" headers="d0e247 ">F</td>
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<td valign="top" width="33.33333333333333%" headers="d0e249 ">26933</td>
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</tr>
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<tr><td valign="top" width="33.33333333333333%" headers="d0e245 ">D21</td>
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<td valign="top" width="33.33333333333333%" headers="d0e247 ">M</td>
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<td valign="top" width="33.33333333333333%" headers="d0e249 ">24720</td>
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</tr>
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<tr><td valign="top" width="33.33333333333333%" headers="d0e245 ">E01</td>
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<td valign="top" width="33.33333333333333%" headers="d0e247 ">M</td>
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<td valign="top" width="33.33333333333333%" headers="d0e249 ">40175</td>
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</tr>
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<tr><td valign="top" width="33.33333333333333%" headers="d0e245 ">E11</td>
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<td valign="top" width="33.33333333333333%" headers="d0e247 ">F</td>
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<td valign="top" width="33.33333333333333%" headers="d0e249 ">22810</td>
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</tr>
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<tr><td valign="top" width="33.33333333333333%" headers="d0e245 ">E11</td>
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<td valign="top" width="33.33333333333333%" headers="d0e247 ">M</td>
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<td valign="top" width="33.33333333333333%" headers="d0e249 ">16545</td>
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</tr>
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<tr><td valign="top" width="33.33333333333333%" headers="d0e245 ">E21</td>
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<td valign="top" width="33.33333333333333%" headers="d0e247 ">F</td>
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<td valign="top" width="33.33333333333333%" headers="d0e249 ">25370</td>
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</tr>
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<tr><td valign="top" width="33.33333333333333%" headers="d0e245 ">E21</td>
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<td valign="top" width="33.33333333333333%" headers="d0e247 ">M</td>
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<td valign="top" width="33.33333333333333%" headers="d0e249 ">23830</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>Because you did not include a WHERE clause in this example, SQL
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examines and process all rows in the CORPDATA.EMPLOYEE table. The rows are
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grouped first by department number and next (within each department) by sex
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before SQL derives the average SALARY value for each group.</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 class="relref"><strong>Related reference</strong><br />
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<div><a href="rbafyorderby.htm" title="The ORDER BY clause specifies the particular order in which you want selected rows returned. The order is sorted by ascending or descending collating sequence of a column's or expression's value.">ORDER BY clause</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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