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<p class="MsoNormal"><span lang="EN-US"> </span></p>
<h4 style="FTEL: 21.45pt"><span lang="EN-US">1. </span><span style="FONT-FAMILY: 黑体">设置主键码</span></h4>
<p class="MsoNormal" style="LINE-HEIGHT: 15.3pt"><span style="FONT-FAMILY: 宋体">在关系数据库中的表中，可以提供一个主键码，该主键码可以基于</span><span lang="EN-US">DataTable</span><span style="FONT-FAMILY: 宋体">中的一个或多个列。</span></p>
<p class="MsoNormal" style="LINE-HEIGHT: 15.3pt"><span style="FONT-FAMILY: 宋体">下面的代码为</span><span lang="EN-US">Product</span><span style="FONT-FAMILY: 宋体">表创建了一个主键码，其模式是前面手工构建的。</span></p>
<p class="MsoNormal" style="LINE-HEIGHT: 15.3pt"><span style="FONT-FAMILY: 宋体">表上的主键码只是约束的一种形式。当主键码添加到</span><span lang="EN-US">DataTable</span><span style="FONT-FAMILY: 宋体">中时，<span style="LETTER-SPACING: 0.1pt">运行</span>库<span style="LETTER-SPACING: 0.1pt">也会对键码列生成一个惟一的约束。这是因为并没有</span></span><span lang="EN-US">PrimaryKey</span><span style="FONT-FAMILY: 宋体">约束类型，主键码是一个或多个列上的惟一约束。</span></p>
<p class="2" style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><a ftel="primarykeys"><span lang="EN-US">public static void ManufacturePrimaryKey(DataTable dt)</span></a></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 13pt; FTEL: 18.45pt"><span lang="EN-US">{</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 13pt; FTEL: 18.45pt"><span lang="EN-US"> DataColumn[] pk = new DataColumn[1];</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 13pt; FTEL: 18.45pt"><span lang="EN-US"> pk[0] = dt.Columns["ProductID"];</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 13pt; FTEL: 18.45pt"><span lang="EN-US"> dt.PrimaryKey = pk;</span></p>
<p class="2" style="MARGIN-TOP: 0cm; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">}</span></p>
<p class="MsoNormal" style="LINE-HEIGHT: 15.6pt"><span style="FONT-FAMILY: 宋体">因为主键码可以包含几个列，所以它可以作为一组</span><span lang="EN-US">DataColumn</span><span style="FONT-FAMILY: 宋体">键入。通过给表的一组列指定属性，就可以给这些列设置主键码。</span></p>
<p class="MsoNormal" style="LINE-HEIGHT: 15.6pt"><span style="FONT-FAMILY: 宋体">要检查表中的约束，可以迭代</span><span lang="EN-US">ConstraintsCollection</span><span style="FONT-FAMILY: 宋体">。上述代码自动生成的约束是</span><span lang="EN-US">Constraint1</span><span style="FONT-FAMILY: 宋体">，它没有什么用，应避免这种情况，最好先在代码中创建约束，然后定义组成主键码的列。</span></p>
<p class="MsoNormal" style="LINE-HEIGHT: 15.6pt"><span style="FONT-FAMILY: 宋体">创建主键码之前，下面的代码给约束命名：</span></p>
<p class="a6" style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">DataColumn[] pk = new DataColumn[1];</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">pk[0] = dt.Columns["ProductID"];</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">dt.Constraints.Add(new UniqueConstraint("PK</span><span lang="EN-US" style="FONT-FAMILY: 'Baskerville BE Regular'">_</span><span lang="EN-US">Products", pk[0]));</span></p>
<p class="a6" style="MARGIN-TOP: 0cm; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">dt.PrimaryKey = pk;</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">惟一约束可以应用到任意多的列上。</span></p>
<h4 style="FTEL: 21.45pt"><span lang="EN-US">2. </span><span style="FONT-FAMILY: 黑体">设置外键码</span></h4>
<p class="MsoNormal" style="LINE-HEIGHT: 15.6pt"><span style="FONT-FAMILY: 宋体">除了惟一约束外，</span><span lang="EN-US">DataTable</span><span style="FONT-FAMILY: 宋体">类还可以包含外键码约束，它们主要用于建立主</span><span lang="EN-US">/</span><span style="FONT-FAMILY: 宋体">从关系，如果正确建立了约束，</span><span lang="EN-US">Foreign Key</span><span style="FONT-FAMILY: 宋体">约束还可以在表之间复制列。在主</span><span lang="EN-US">/</span><span style="FONT-FAMILY: 宋体">从关系的表中，常常有一个父记录</span><span lang="EN-US">(Order)</span><span style="FONT-FAMILY: 宋体">和许多子记录</span><span lang="EN-US">(Order Line)</span><span style="FONT-FAMILY: 宋体">，它们是通过父记录的主键码链接起来的。</span></p>
<p class="MsoNormal" style="LINE-HEIGHT: 15.6pt"><span style="FONT-FAMILY: 宋体">外键码约束只能用于同一个</span><span lang="EN-US">DataSet</span><span style="FONT-FAMILY: 宋体">中的表，所以下面的示例使用</span><span lang="EN-US">Northwind</span><span style="FONT-FAMILY: 宋体">数据库中的</span><span lang="EN-US">Categories</span><span style="FONT-FAMILY: 宋体">表，给该表和</span><span lang="EN-US">Products</span><span style="FONT-FAMILY: 宋体">表之间指定约束，如图</span><span lang="EN-US">21-8</span><span style="FONT-FAMILY: 宋体">所示。</span></p>
<p align="center"><span lang="EN-US"><img height="135" src="21/image008.jpg" width="318" alt="" /></span></p>
<p style="FTEL: 8.15pt" align="center"><span style="FONT-FAMILY: 宋体">图</span><span lang="EN-US"> 21-8</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">第一步是为</span><span lang="EN-US">Categories</span><span style="FONT-FAMILY: 宋体">表生成一个新的数据表：</span></p>
<p class="2" style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">DataTable categories = new DataTable("Categories");</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">categories.Columns.Add(new DataColumn("CategoryID", typeof(int)));</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">categories.Columns.Add(new DataColumn("CategoryName", typeof(string)));</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">categories.Columns.Add(new DataColumn("Description", typeof(string)));</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">categories.Constraints.Add(new UniqueConstraint("PK</span><span lang="EN-US" style="FONT-FAMILY: 'Baskerville BE Regular'">_</span><span lang="EN-US">Categories",</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> categories.Columns["CategoryID"]));</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">categories.PrimaryKey = new DataColumn[1]</span></p>
<p class="2" style="MARGIN-TOP: 0cm; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US"> {categories.Columns["CategoryID"]};</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">上述代码的最后一行为</span><span lang="EN-US">Categories</span><span style="FONT-FAMILY: 宋体">表创建了主键码。在本例中，主键码是一个单列，但可以使用数组语法在多个列上生成一个键码。</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">然后，需要在两个表之间创建约束：</span></p>
<p class="2" style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">DataColumn parent = ds.Tables["Categories"].Columns["CategoryID"];</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">DataColumn child = ds.Tables["Products"].Columns["CategoryID"];</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">ForeignKeyConstraint fk = </span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> new ForeignKeyConstraint("FK</span><span lang="EN-US" style="FONT-FAMILY: 'Baskerville BE Regular'">_</span><span lang="EN-US">Product</span><span lang="EN-US" style="FONT-FAMILY: 'Baskerville BE Regular'">_</span><span lang="EN-US">CategoryID", parent, child);</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">fk.UpdateRule = Rule.Cascade;</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">fk.DeleteRule = Rule.SetNull;</span></p>
<p class="2" style="MARGIN-TOP: 0cm; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">ds.Tables["Products"].Constraints.Add(fk);</span></p>
<p class="MsoNormal" style="LINE-HEIGHT: 18pt"><span style="FONT-FAMILY: 宋体">这个约束应用到</span><span lang="EN-US">Categories.CategoryID</span><span style="FONT-FAMILY: 宋体">和</span><span lang="EN-US">Products.CategoryID</span><span style="FONT-FAMILY: 宋体">之间的链接上。有</span><span lang="EN-US">4</span><span style="FONT-FAMILY: 宋体">个不同的</span><span lang="EN-US">ForeignKeyConstraint</span><span style="FONT-FAMILY: 宋体">，但应使用可以给约束命名的</span><span lang="EN-US">ForeignKeyConstraint</span><span style="FONT-FAMILY: 宋体">。</span></p>
<h4 style="FTEL: 21.45pt"><span lang="EN-US">3. </span><span style="FONT-FAMILY: 黑体">设置更新和删除约束</span></h4>
<p class="MsoNormal" style="LINE-HEIGHT: 18pt"><span style="FONT-FAMILY: 宋体">除了在父表和子表之间定义约束之外，还可以在更新约束中的一个列时定义应执行的操作。</span></p>
<p class="MsoNormal" style="LINE-HEIGHT: 18pt"><span style="FONT-FAMILY: 宋体">上面的示例设置了更新规则和删除规则，在对父表中的列</span><span lang="EN-US">(</span><span style="FONT-FAMILY: 宋体">或行</span><span lang="EN-US">)</span><span style="FONT-FAMILY: 宋体">执行某种操作时，使用这些规则，以确定应对子表中的行进行什么操作。通过</span><span lang="EN-US">Rule</span><span style="FONT-FAMILY: 宋体">枚举可以应用</span><span lang="EN-US">4</span><span style="FONT-FAMILY: 宋体">种不同的规则：</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">Cascade</span><span style="FONT-FAMILY: 宋体; LETTER-SPACING: -1pt">——</span><span style="LETTER-SPACING: -1pt"> </span><span style="FONT-FAMILY: 宋体; LETTER-SPACING: 0.1pt">如果更新了父键，就应把新的键值复制到所有的子记录上。如果删除了父记</span><span style="FONT-FAMILY: 宋体">录，也将删除子记录，这是默认选项。</span></p>
<p class="1" style="MARGIN-LEFT: 37.55pt; FTEL: -16.1pt"><span lang="EN-US" style="LETTER-SPACING: 0.1pt">●<span style="FONT: 7pt 'Times New Roman'"> </span></span><span lang="EN-US">None</span><span style="FONT-FAMILY: 宋体; LETTER-SPACING: -1pt">——</span><span style="LETTER-SPACING: -1pt"> </span><span style="FONT-FAMILY: 宋体">不执行任何操作，这个选项会留下子数据表中的孤立行。</span></p>
<p class="1" style="MARGIN-LEFT: 37.55pt; FTEL: -16.1pt"><span lang="EN-US" style="LETTER-SPACING: 0.1pt">●<span style="FONT: 7pt 'Times New Roman'"> </span></span><span lang="EN-US">SetDefault</span><span style="FONT-FAMILY: 宋体; LETTER-SPACING: -1pt">——</span><span style="LETTER-SPACING: -1pt"> </span><span style="FONT-FAMILY: 宋体; LETTER-SPACING: 0.1pt">如果定义了一个子记录，那么每个受影响的子记录都把外键码列设置为其默认值。</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">SetNull</span><span style="FONT-FAMILY: 宋体; LETTER-SPACING: -1pt">——</span><span style="LETTER-SPACING: -1pt"> </span><span style="FONT-FAMILY: 宋体; LETTER-SPACING: 0.1pt">所有的子行都把主列设置为</span><span lang="EN-US" style="LETTER-SPACING: 0.1pt">DBNull</span><span style="FONT-FAMILY: 宋体; LETTER-SPACING: 0.1pt">。</span><span lang="EN-US" style="LETTER-SPACING: 0.1pt">(</span><span style="FONT-FAMILY: 宋体; LETTER-SPACING: 0.1pt">按照</span><span lang="EN-US" style="LETTER-SPACING: 0.1pt">Microsoft</span><span style="FONT-FAMILY: 宋体; LETTER-SPACING: 0.1pt">选择的命名约定，主列应是</span><span lang="EN-US" style="LETTER-SPACING: 0.1pt">SetDBNull</span><span style="FONT-FAMILY: 宋体; LETTER-SPACING: 0.1pt">。</span><span lang="EN-US">)</span></p>
<p class="a3" style="MARGIN-TOP: 8.15pt; FTEL: 21.45pt"><span style="FONT-FAMILY: 黑体">提示：</span></p>
<p class="a1" style="FTEL: 21.45pt"><span style="FONT-FAMILY: 楷体_GB2312">如果</span><span lang="EN-US">EnforceConstraints</span><span style="FONT-FAMILY: 楷体_GB2312">属性为</span><span lang="EN-US">true</span><span style="FONT-FAMILY: 楷体_GB2312">，约束就只能在</span><span lang="EN-US">DataSet</span><span style="FONT-FAMILY: 楷体_GB2312">中使用。</span></p>
<p class="MsoNormal" style="LINE-HEIGHT: 18pt"><span style="FONT-FAMILY: 宋体">前面介绍了组成</span><span lang="EN-US">DataSet</span><span style="FONT-FAMILY: 宋体">中约束部分的主类，解释了如何在代码中手工生成这些类。还可以使用</span><span lang="EN-US">.NET</span><span style="FONT-FAMILY: 宋体">附带的</span><span lang="EN-US">XML</span><span style="FONT-FAMILY: 宋体">模式文件和</span><span lang="EN-US">XSD</span><span style="FONT-FAMILY: 宋体">工具定义</span><span lang="EN-US">DataTable</span><span style="FONT-FAMILY: 宋体">、</span><span lang="EN-US">DataRow</span><span style="FONT-FAMILY: 宋体">、</span><span lang="EN-US">DataColumn</span><span style="FONT-FAMILY: 宋体">、</span><span lang="EN-US">DataRelation </span><span style="FONT-FAMILY: 宋体">和</span><span lang="EN-US">Constraint</span><span style="FONT-FAMILY: 宋体">。下一节将说明如何建立一个简单的模式，以及如何生成类型安全的类，以访问数据。</span></p></div>
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<h1 id="divCurrentNode2" style="color: #b83507; width: 100%; text-align: left; font-size: 12px; padding-left: 2px">当前章节:<a href='100165607.htm'><font color='red'>21.5.4 数据约束</font></a></h1>
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<div style='float:left;width:49%'>·<a href='100165604.htm'>21.5.1 数据表</a></div><div style='float:right;width:49%'>·<a href='100165605.htm'>21.5.2 数据列</a></div><div style='float:left;width:49%'>·<a href='100165606.htm'>21.5.3 数据关系</a></div><div style='float:right;width:49%'>·<a href='100165608.htm'>21.6 XML模式</a></div><div style='float:left;width:49%'>·<a href='100165609.htm'>21.7 填充数据集</a></div><div style='float:right;width:49%'>·<a href='100165610.htm'>21.7.1 用数据适配器来填充 DataSet</a></div></div>
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