uTools-Manuals/docs/sql/UNION 操作符.html

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<h1>SQL UNION 操作符</h1>
<p class="intro">SQL UNION 操作符合并两个或多个 SELECT 语句的结果。</p> <h2>SQL UNION 操作符</h2> <p>UNION 操作符用于合并两个或多个 SELECT 语句的结果集。</p> <p>请注意，UNION 内部的每个 SELECT 语句必须拥有相同数量的列。列也必须拥有相似的数据类型。同时，每个 SELECT 语句中的列的顺序必须相同。</p> <h3>SQL UNION 语法</h3> <div class="code notranslate"><pre><div> SELECT <em>column_name(s)</em> FROM <em>table1</em><br/> UNION<br/> SELECT <em>column_name(s)</em> FROM <em>table2</em>;</div></pre></div> <p><b>注释：</b>默认地，UNION 操作符选取不同的值。如果允许重复的值，请使用 UNION ALL。</p> <h3>SQL UNION ALL 语法</h3> <div class="code notranslate"><pre><div> SELECT <em>column_name(s)</em> FROM <em>table1</em><br/> UNION ALL<br/> SELECT <em>column_name(s)</em> FROM <em>table2</em>;</div></pre></div> <p><b>注释：</b>UNION 结果集中的列名总是等于 UNION 中第一个 SELECT 语句中的列名。</p> <h2>演示数据库</h2> <p>在本教程中，我们将使用众所周知的 Northwind 样本数据库。</p> <p>下面是选自 "Customers" 表的数据：</p> <table class="reference notranslate">
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<th width="15%">CustomerID</th> <th>CustomerName</th> <th>ContactName</th> <th>Address</th> <th>City</th> <th>PostalCode</th> <th>Country</th> </tr>
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<td>1<br/><br/>
</td> <td>Alfreds Futterkiste</td> <td>Maria Anders</td> <td>Obere Str. 57</td> <td>Berlin</td> <td>12209</td> <td>Germany</td> </tr>
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<td>2</td> <td>Ana Trujillo Emparedados y helados</td> <td>Ana Trujillo</td> <td>Avda. de la Constitución 2222</td> <td>México D.F.</td> <td>05021</td> <td>Mexico</td> </tr>
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<td>3</td> <td>Antonio Moreno Taquería</td> <td>Antonio Moreno</td> <td>Mataderos 2312</td> <td>México D.F.</td> <td>05023</td> <td>Mexico</td> </tr>
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<p>选自 "Suppliers" 表的数据：</p> <table class="reference notranslate">
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<th width="15%">SupplierID</th> <th>SupplierName</th> <th>ContactName</th> <th>Address</th> <th>City</th> <th>PostalCode</th> <th>Country</th> </tr>
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<td>1</td> <td>Exotic Liquid</td> <td>Charlotte Cooper</td> <td>49 Gilbert St.</td> <td>Londona</td> <td>EC1 4SD</td> <td>UK</td> </tr>
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<td>2</td> <td>New Orleans Cajun Delights</td> <td>Shelley Burke</td> <td>P.O. Box 78934</td> <td>New Orleans</td> <td>70117</td> <td>USA</td> </tr>
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<td>3</td> <td>Grandma Kelly's Homestead</td> <td>Regina Murphy</td> <td>707 Oxford Rd.</td> <td>Ann Arbor</td> <td>48104</td> <td>USA</td> </tr>
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<h2>SQL UNION 实例</h2> <p>下面的 SQL 语句从 "Customers" 和 "Suppliers" 表中选取所有<strong>不同的</strong>城市（只有不同的值）：</p> <div class="example margin-b-10"> <h2 class="example">实例</h2> <pre><div class="example_code notranslate"> SELECT City FROM Customers<br/>UNION<br/>SELECT City FROM Suppliers<br/>ORDER BY City; </div></pre> </div> <p><b>注释：</b>UNION 不能用于列出两个表中所有的城市。如果一些客户和供应商来自同一个城市，每个城市只会列出一次。UNION 只会选取不同的值。请使用 UNION ALL 来选取重复的值！</p> <h2>SQL UNION ALL 实例</h2> <p>下面的 SQL 语句使用 UNION ALL 从 "Customers" 和 "Suppliers" 表中选取<strong>所有的</strong>城市（也有重复的值）：</p> <div class="example margin-b-10"> <h2 class="example">实例</h2> <pre><div class="example_code notranslate"> SELECT City FROM Customers<br/>UNION ALL<br/>SELECT City FROM Suppliers<br/> ORDER BY City; </div></pre> </div> <h2>带有 WHERE 的 SQL UNION ALL</h2> <p>下面的 SQL 语句使用 UNION ALL 从 "Customers" 和 "Suppliers" 表中选取<strong>所有的</strong>德国城市（也有重复的值）：</p> <div class="example margin-b-10"> <h2 class="example">实例</h2> <pre><div class="example_code notranslate"> SELECT City, Country FROM Customers<br/>WHERE Country='Germany'<br/>UNION ALL<br/> SELECT City, Country FROM Suppliers<br/>WHERE Country='Germany'<br/>ORDER BY City; </div></pre> </div> <div class="text-center padding-10 margin-t-5">
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