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15 lines
2.6 KiB
HTML
15 lines
2.6 KiB
HTML
<div class="m-bg">
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<h1>SQL COUNT() 函数</h1>
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<p class="intro">COUNT() 函数返回匹配指定条件的行数。</p> <h3>SQL COUNT(column_name) 语法</h3> <p>COUNT(column_name) 函数返回指定列的值的数目（NULL 不计入）：</p> <div class="code notranslate"><pre><code class="language-sql"><div> SELECT COUNT(column_name) FROM table_name;</div></code></pre></div> <h3>SQL COUNT(*) 语法</h3> <p>COUNT(*) 函数返回表中的记录数：</p> <div class="code notranslate"><pre><code class="language-sql"><div> SELECT COUNT(*) FROM table_name;</div></code></pre></div> <h3>SQL COUNT(DISTINCT column_name) 语法</h3> <p>COUNT(DISTINCT column_name) 函数返回指定列的不同值的数目：</p> <div class="code notranslate"><pre><code class="language-sql"><div> SELECT COUNT(DISTINCT column_name) FROM table_name;</div></code></pre></div> <p><b>注释：</b>COUNT(DISTINCT) 适用于 ORACLE 和 Microsoft SQL Server，但是无法用于 Microsoft Access。</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 "Orders" 表的数据：</p> <table class="reference notranslate">
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<tr>
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<th>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>10265</td> <td>7</td> <td>2</td> <td>1996-07-25</td> <td>1</td> </tr>
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<tr>
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<td>10266</td> <td>87</td> <td>3</td> <td>1996-07-26</td> <td>3</td> </tr>
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<tr>
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<td>10267</td> <td>25</td> <td>4</td> <td>1996-07-29</td> <td>1</td> </tr>
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</table>
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<h2>SQL COUNT(column_name) 实例</h2> <p>下面的 SQL 语句计算 "Orders" 表中 "CustomerID"=7 的订单数：</p> <div class="example margin-b-10"> <h2 class="example">实例</h2> <pre><code class="language-sql"><div class="example_code notranslate"> SELECT COUNT(CustomerID) AS OrdersFromCustomerID7 FROM Orders<br/>WHERE CustomerID=7; </div></code></pre> </div> <h2>SQL COUNT(*) 实例</h2> <p>下面的 SQL 语句计算 "Orders" 表中的订单总数：</p> <div class="example margin-b-10"> <h2 class="example">实例</h2> <pre><code class="language-sql"><div class="example_code notranslate"> SELECT COUNT(*) AS NumberOfOrders FROM Orders; </div></code></pre> </div> <h2>SQL COUNT(DISTINCT column_name) 实例</h2> <p>下面的 SQL 语句计算 "Orders" 表中不同客户的数目：</p> <div class="example margin-b-10"> <h2 class="example">实例</h2> <pre><code class="language-sql"><div class="example_code notranslate"> SELECT COUNT(DISTINCT CustomerID) AS NumberOfCustomers FROM Orders; </div></code></pre> </div> <br/><br/><div class="text-center padding-10 margin-t-5">
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