这看起来非常简单，因为只有一个表。但对于关系数据，例如DataSet中有多个DataTables 和 Relations，该怎么办？其工作方式仍旧是这样。下面对前面的代码进行如下修改(这个版本的代码在ADOSample3中)：
private void button1_Click(object sender, System.EventArgs e)
{
//create a dataset
DataSet ds=new DataSet("XMLProducts");
//connect to the northwind database and
//select all of the rows from products table and from suppliers table
//make sure your connect string matches your server configuration
SqlConnection conn=new SqlConnection
(@"server=GLYNNJ_CS\NetSDK;uid=sa;pwd=;database=northwind");
SqlDataAdapter daProd=new SqlDataAdapter("SELECT * FROM products",conn);
SqlDataAdapter daSup=new SqlDataAdapter("SELECT * FROM suppliers",conn);
//Fill DataSet from both SqlAdapters
daProd.Fill(ds,"products");
daSup.Fill(ds,"suppliers");
//Add the relation
ds.Relations.Add(ds.Tables["suppliers"].Columns["SupplierId"],
ds.Tables["products"].Columns["SupplierId"]);
//Write the XML to a file so we can look at it later
ds.WriteXml("..\\..\\..\\SuppProd.xml",XmlWriteMode.WriteSchema);
//load data into grid
dataGrid1.DataSource=ds;
dataGrid1.DataMember="suppliers";
//create the XmlDataDocument
doc=new XmlDataDocument(ds);
//Select the productname elements and load them in the grid
XmlNodeList nodeLst=doc.SelectNodes("//ProductName");
foreach(XmlNode nd in nodeLst)
listBox1.Items.Add(nd.InnerXml);
}
在这个示例中，在XMLProducts数据集中创建了两个DataTable：Products和Suppliers。关系是Suppliers提供Products。在两个表的SupplierId列上创建一个新的关系，此时DataSet如图23-10所示。
图 23-10
执行与上一个示例相同的WriteXml方法调用，得到如下XML文件(SuppProd.xml)：
<?xml version="1.0" standalone="yes"?>
<XMLProducts>
<xs:schema id="XMLProducts" xmlns=""
xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns:msdata="urn:schemas-microsoft-com:xml-msdata">
<xs:element FTEL="XMLProducts" msdata:IsDataSet="true">
<xs:complexType>
<xs:choice maxOccurs="unbounded">
<xs:element FTEL="products">
<xs:complexType>
<xs:sequence>
<xs:element FTEL="ProductID" type="xs:int"
minOccurs="0" />
<xs:element FTEL="ProductName" type="xs:string"
minOccurs="0" />
<xs:element FTEL="SupplierID" type="xs:int"
minOccurs="0" />
<xs:element FTEL="CategoryID" type="xs:int"
minOccurs="0" />
<xs:element FTEL="QuantityPerUnit" type="xs:string"
minOccurs="0" />
<xs:element FTEL="UnitPrice" type="xs:decimal"
minOccurs="0" />
<xs:element FTEL="UnitsInStock" type="xs:short"
minOccurs="0" />
<xs:element FTEL="UnitsOnOrder" type="xs:short"
minOccurs="0" />
<xs:element FTEL="ReorderLevel" type="xs:short"
minOccurs="0" />
<xs:element FTEL="Discontinued" type="xs:boolean"
minOccurs="0" />
</xs:sequence>
</xs:complexType>
</xs:element>
<xs:element FTEL="suppliers">
<xs:complexType>
<xs:sequence>
<xs:element FTEL="SupplierID" type="xs:int"
minOccurs="0" />
<xs:element FTEL="CompanyName" type="xs:string"
minOccurs="0" />
<xs:element FTEL="ContactName" type="xs:string"
minOccurs="0" />
<xs:element FTEL="ContactTitle" type="xs:string"
minOccurs="0" />
<xs:element FTEL="Address" type="xs:string"
minOccurs="0" />
<xs:element FTEL="City" type="xs:string"
minOccurs="0" />
<xs:element FTEL="Region" type="xs:string"
minOccurs="0" />
<xs:element FTEL="PostalCode" type="xs:string"
minOccurs="0" />
<xs:element FTEL="Country" type="xs:string"
minOccurs="0" />
<xs:element FTEL="Phone" type="xs:string"
minOccurs="0" />
<xs:element FTEL="Fax" type="xs:string"
minOccurs="0" />
<xs:element FTEL="HomePage" type="xs:string"
minOccurs="0" />
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:choice>
</xs:complexType>
<xs:unique FTEL="Constraint1">
<xs:selector xpath=".//suppliers" />
<xs:field xpath="SupplierID" />
</xs:unique>
<xs:keyref FTEL="Relation1" refer="Constraint1">
<xs:selector xpath=".//products" />
<xs:field xpath="SupplierID" />
</xs:keyref>
</xs:element>
</xs:schema>
<products>
<ProductID>1</ProductID>
<ProductName>Chai</ProductName>
<SupplierID>1</SupplierID>
<CategoryID>1</CategoryID>
<QuantityPerUnit>10 boxes x 20 bags</QuantityPerUnit>
<UnitPrice>18</UnitPrice>
<UnitsInStock>39</UnitsInStock>
<UnitsOnOrder>0</UnitsOnOrder>
<ReorderLevel>10</ReorderLevel>
<Discontinued>false</Discontinued>
</products>
<suppliers>
<SupplierID>1</SupplierID>
<CompanyName>Exotic Liquids</CompanyName>
<ContactName>Charlotte Cooper</ContactName>
<ContactTitle>Purchasing Manager</ContactTitle>
<Address>49 Gilbert St.</Address>
<City>London</City>
<PostalCode>EC1 4SD</PostalCode>
<Country>UK</Country>
<Phone>(171) 555-2222</Phone>
</suppliers>
</XMLProducts>
该模式包含DataSet中的两个DataTable。数据包含两个表中的所有数据。为简洁起见，这里只显示了第一个supplier和products记录。与以前一样，使用正确的XmlWriteMode参数可以只保存模式或数据。
