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26 lines
2.9 KiB
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26 lines
2.9 KiB
HTML
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<h1>SQL LEFT JOIN 关键字</h1>
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<h2>SQL LEFT JOIN 关键字</h2> <p>LEFT JOIN 关键字从左表（table1）返回所有的行，即使右表（table2）中没有匹配。如果右表中没有匹配，则结果为 NULL。</p> <h3>SQL LEFT JOIN 语法</h3> <div class="code notranslate"><pre><code class="language-sql"><div> SELECT <em>column_name(s)</em><br/> FROM <em>table1</em><br/> LEFT JOIN <em>table2</em><br/> ON <em>table1.column_name</em>=<em>table2.column_name</em>;</div></code></pre></div> <p>或：</p> <div class="code notranslate"><pre><code class="language-sql"><div> SELECT <em>column_name(s)</em><br/> FROM <em>table1</em><br/> LEFT OUTER JOIN <em>table2</em><br/> ON <em>table1.column_name</em>=<em>table2.column_name</em>;</div></code></pre></div> <p><b>注释：</b>在某些数据库中，LEFT JOIN 称为 LEFT OUTER JOIN。</p> <p style="text-align:center"><img alt="SQL LEFT JOIN" height="145" src="../image/5615cc03dc434.gif" width="200"/></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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<tr>
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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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<tr>
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<td>1<br/><br/>
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</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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<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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<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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</table>
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<p>选自 "Orders" 表的数据：</p> <table class="reference notranslate">
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<tr>
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<th width="15%">OrderID</th> <th>CustomerID</th> <th>EmployeeID</th> <th>OrderDate</th> <th>ShipperID</th> </tr>
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<tr>
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<td>10308</td> <td>2</td> <td>7</td> <td>1996-09-18</td> <td>3</td> </tr>
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<tr>
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<td>10309</td> <td>37</td> <td>3</td> <td>1996-09-19</td> <td>1</td> </tr>
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<tr>
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<td>10310</td> <td>77</td> <td>8</td> <td>1996-09-20</td> <td>2</td> </tr>
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</table>
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<h2>SQL LEFT JOIN 实例</h2> <p>下面的 SQL 语句将返回所有客户及他们的订单（如果有的话）：</p> <div class="example margin-b-10"> <h2 class="example">实例</h2> <pre><code class="language-sql"><div class="example_code notranslate"> SELECT Customers.CustomerName, Orders.OrderID<br/> FROM Customers<br/> LEFT JOIN Orders<br/> ON Customers.CustomerID=Orders.CustomerID<br/> ORDER BY Customers.CustomerName; </div></code></pre> </div> <p><strong>注释：</strong>LEFT JOIN 关键字从左表（Customers）返回所有的行，即使右表（Orders）中没有匹配。</p> <div class="text-center padding-10 margin-t-5">
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