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<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 14.5pt; FTEL: 18.45pt"><span lang="EN-GB"> </span><span lang="EN-US">while(reader.Read())</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 14.5pt; FTEL: 18.45pt"><span lang="EN-GB"> </span><span lang="EN-US">{</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 14.5pt; FTEL: 18.45pt"><span lang="EN-US"> </span><span lang="EN-GB"> </span><span lang="EN-US">Console.WriteLine("Contact : {0,-20} Company : {1}" , </span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 14.5pt; FTEL: 18.45pt"><span lang="EN-US"> reader[0] , reader[1]);</span><span class="MsoCommentReference"><span lang="EN-US" style="DISPLAY: none; FONT-SIZE: 8pt; FONT-FAMILY: 'Baskerville BE Regular'; LETTER-SPACING: 0.2pt"> </span></span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 14.5pt; FTEL: 18.45pt"><span lang="EN-GB"> </span><span lang="EN-US">}</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 14.5pt; FTEL: 18.45pt"><span lang="EN-US"> }</span></p>
<p class="a6" style="MARGIN-TOP: 0cm; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">}</span></p>
<p align="center"><span lang="EN-US"><img height="202" src="21/image002.jpg" width="403" alt="" /></span></p>
<p style="FTEL: 8.15pt" align="center"><span style="FONT-FAMILY: 宋体">图</span><span lang="EN-US"> 21-2</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">本章的后面将讨论</span><span lang="EN-US"><provider>DataReader</span><span style="FONT-FAMILY: 宋体">对象。</span></p>
<h4 style="FTEL: 21.45pt"><span lang="EN-GB">3.</span><span lang="EN-GB" style="FONT-FAMILY: 宋体"> </span><span lang="EN-GB">ExecuteScalar()</span><span style="FONT-FAMILY: 黑体">方法</span></h4>
<p class="MsoNormal"><a ftel="ExecuteScalar"><span style="FONT-FAMILY: 宋体">在许多情况下，需要从</span><span lang="EN-US">SQL</span></a><span style="FONT-FAMILY: 宋体">语句返回一个结果，例如给定表中的记录个数，或者服务器的当前日期</span><span lang="EN-US">/</span><span style="FONT-FAMILY: 宋体">时间。</span><span lang="EN-US">ExecuteScalar</span><span style="FONT-FAMILY: 宋体">方法就可以用于这些场合：</span></p>
<p class="a6" style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-GB">using System;</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-GB">using System.Data.SqlClient;</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-GB">public class ExecuteScalarExample</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-GB">{</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-GB"> public static void Main(string[] args)</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-GB"> {</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-GB"> string source ="server=(local)\\NetSDK;" +</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-GB"> "</span><span lang="EN-GB"> </span><span lang="EN-GB">integrated security=SSPI;" + </span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-GB"> "database=Northwind";</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-GB"> string select = "SELECT COUNT(*) FROM Customers";</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-GB"> SqlConnection conn = new SqlConnection(source);</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-GB"> conn.Open();</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-GB"> SqlCommand cmd = new SqlCommand(select, conn);</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-GB"> object o = cmd.ExecuteScalar();</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-GB"> Console.WriteLine ( o ) ;</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-GB"> }</span></p>
<p class="a6" style="MARGIN-TOP: 0cm; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-GB">}</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">该方法返回一个对象，如果需要，可以把该对象的数据类型转换为合适的类型。</span></p>
<h4 style="FTEL: 21.45pt"><span lang="EN-GB">4. ExecuteXmlReader()</span><span style="FONT-FAMILY: 黑体">方法</span><span lang="EN-GB">(</span><span style="FONT-FAMILY: 黑体">只用于</span><span lang="EN-GB">SqlC</span><span lang="EN-US">lient</span><span style="FONT-FAMILY: 黑体">提供程序</span><span lang="EN-GB">)</span></h4>
<p class="MsoNormal"><a ftel="ExecuteXMLReader"><span style="FONT-FAMILY: 宋体">顾名思义</span></a><span style="FONT-FAMILY: 宋体">，</span><span style="FONT-FAMILY: 宋体">这个方法执行命令</span><span style="FONT-FAMILY: 宋体">，</span><span style="FONT-FAMILY: 宋体">给调用者返回一个</span><span lang="EN-US">XmlReader</span><span style="FONT-FAMILY: 宋体">对象。</span><span lang="EN-US">SQL Server</span><span style="FONT-FAMILY: 宋体">允许使用</span><span lang="EN-US">FOR XML</span><span style="FONT-FAMILY: 宋体">子句来扩展</span><span lang="EN-US">SQL</span><span style="FONT-FAMILY: 宋体">子句</span><span style="FONT-FAMILY: 宋体">，</span><span style="FONT-FAMILY: 宋体">这个子句可以带有下述</span><span lang="EN-US">3</span><span style="FONT-FAMILY: 宋体">个选项中的一个</span><span style="FONT-FAMILY: 宋体">：</span></p>
<p class="1" style="MARGIN-LEFT: 37.55pt; FTEL: -16.1pt"><span lang="EN-US">●<span style="FONT: 7pt 'Times New Roman'"> </span></span><span lang="EN-US">FOR XML AUTO</span><span style="FONT-FAMILY: 宋体">：根据</span><span lang="EN-US">FROM</span><span style="FONT-FAMILY: 宋体">子句中的表建立一个树</span></p>
<p class="1" style="MARGIN-LEFT: 37.55pt; FTEL: -16.1pt"><span lang="EN-US">●<span style="FONT: 7pt 'Times New Roman'"> </span></span><span lang="EN-US">FOR XML RAW</span><span style="FONT-FAMILY: 宋体">：结果集中的行映射为元素，其中的列映射为属性</span></p>
<p class="1" style="MARGIN-LEFT: 37.55pt; FTEL: -16.1pt"><span lang="EN-US">●<span style="FONT: 7pt 'Times New Roman'"> </span></span><span lang="EN-US">FOR XML EXPLICIT</span><span style="FONT-FAMILY: 宋体">：必须指定要返回的</span><span lang="EN-US">XML</span><span style="FONT-FAMILY: 宋体">树的形状</span></p>
<p class="MsoNormal"><em><span lang="EN-US">Professional SQL Server 2000 XML</span></em><span lang="EN-US">(ISBN 1-861005-46-6)</span><span style="FONT-FAMILY: 宋体">一书列出了这些选项的完整描述。下面的示例使用了</span><span lang="EN-US">AUTO</span><span style="FONT-FAMILY: 宋体">：</span></p>
<p class="a6" style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-GB">using System;</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-GB">using System.Data.SqlClient;</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-GB">using System.Xml;</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-GB">public class ExecuteXmlReade</span><span lang="EN-US">rExample</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-GB">{</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-GB"> public static void Main(string[] args)</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-GB"> {</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-GB"> string source ="server=(local)\\NetSDK;" +</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-GB"> "</span><span lang="EN-GB"> </span><span lang="EN-GB">integrated security=SSPI;" + </span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-GB"> "database=Northwind";</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-GB"> </span><span lang="EN-US">string select = "SELECT ContactName,CompanyName " +</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-GB"> </span><span lang="EN-US"> "FROM Customers FOR XML AUTO";</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-GB"> </span><span lang="EN-US">SqlConnection conn = new SqlConnection(source);</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-GB"> </span><span lang="EN-US">conn.Open();</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-GB"> </span><span lang="EN-US">SqlCommand cmd = new SqlCommand(select, conn);</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-GB"> </span><span lang="EN-US">XmlReader xr = cmd.ExecuteXmlReader();</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 48.65pt"><span lang="EN-US">xr.Read()</span><span style="FONT-FAMILY: 宋体">；</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 48.65pt"><span lang="EN-US">string s;</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-GB"> do</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-GB"> </span><span lang="EN-US">{</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> </span><span lang="EN-GB"> s = xr.</span><span lang="EN-US"> ReadOuterXml();</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> if (s!="")</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> Console.WriteLine(s);</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-GB"> </span><span lang="EN-US">} while (s!="");</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-GB"> </span><span lang="EN-US">conn.Close();</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> }</span></p>
<p class="a6" style="MARGIN-TOP: 0cm; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">}</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">注意必须导入</span><span lang="EN-US">System.Xml</span><span style="FONT-FAMILY: 宋体">命名空间，才能输出返回的</span><span lang="EN-US">XML</span><span style="FONT-FAMILY: 宋体">。这个命名空间和</span><span lang="EN-US">.NET Framework</span><span style="FONT-FAMILY: 宋体">其他的</span><span lang="EN-US">XML</span><span style="FONT-FAMILY: 宋体">功能将在第</span><span lang="EN-US">24</span><span style="FONT-FAMILY: 宋体">章中详细论述。</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">本例在</span><span lang="EN-US">SQL</span><span style="FONT-FAMILY: 宋体">语句中包含了</span><span lang="EN-US">FOR XML AUTO</span><span style="FONT-FAMILY: 宋体">子句，然后调用</span><span lang="EN-US">ExecuteXmlReader()</span><span style="FONT-FAMILY: 宋体">方法。代码的结果图如图</span><span lang="EN-US">21-3</span><span style="FONT-FAMILY: 宋体">所示。</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">在</span><span lang="EN-US">SQL</span><span style="FONT-FAMILY: 宋体">子句中，我们指定了</span><span lang="EN-US">FROM</span><span lang="EN-US"> </span><span lang="EN-US">Customers</span><span style="FONT-FAMILY: 宋体">，这样类型</span><span lang="EN-US">Customers</span><span style="FONT-FAMILY: 宋体">的元素就显示在输出中。为它添加元素，每个元素对应于从数据库中选择出来的列。这就为每个从数据库中选择出来的行建立了</span><span lang="EN-US">XML</span><span style="FONT-FAMILY: 宋体">标志。</span></p>
<p align="center"><span lang="EN-US"><img height="207" src="21/image003.jpg" width="406" alt="" /></span></p>
<p style="FTEL: 8.15pt" align="center"><span style="FONT-FAMILY: 宋体">图</span><span lang="EN-US"> 21-3</span></p></div>
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