Mapping DataSet to XML and backwards

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Published: 10 Sep 2007
By: Xun Ding

This article illustrates the translation to and from an ADO .NET DataSet and XML.

Introduction

Since the arrival of the Internet, people have become more and more dependent on it for daily operations. The Web's (as well as computing applications') insatiable appetite for data has grown even more intense. Most websites have seen the interplay of relational database and XML. Websites rely on relational databases for storing a vast amount of data and performing fast retrieval and manipulation. For exchanging data, they opt for protocols such as SOAP or REST (what strange names!). In these cases, XML serves both as the foundation and the ultimate representation form. Let's briefly look at the origin of XML and make a skim comparison between a relational database and the XML data model.

A brief history of XML

Relational database was born out of the ideas of an IBM mathematician and computer scientist, E. B. Codd, in the early 1970s. Since then it has become the "IT" player in the database industry. In comparison, XML is the new kid in town and it is the result of a never-ending quest for a simpler yet more powerful language for data storage and communication.

In 1986, SGML or "Standard Generalized Markup Language" was issued as an international standard. It is a complex and somewhat rigid language that requires expensive authoring tools. SGML was used extensively in large government, legal and publishing industries. In 1990, a simpler and more flexible language called HTML was created and it soon became enormously popular. However, its shortcomings also became increasingly visible. In 1996, the World Wide Web Consortium (W3C) (underwritten by Sun and other outside organizations) formed a committee to explore ways of harnessing the strength of HTML and SGML while sidestepping its limitations. The result is the creation of XML.

Relational Database and XML data model

A relational database consists of a collection of two-dimensional tables of rows and columns, interconnected by keys shared across tables. Through relations (shared keys), tables can be joined, queried and rearranged in different ways. There is no hierarchy or sequence to speak of.XML on the other hand is all about hierarchy and sequence, as it is mirrored in such terms, parents, children and siblings (it might as well be superiors, subordinates and colleagues).

Since its debut, XML has wooed techies and laymen alike with its easiness and flexibility; and the fact that it is human-readable. However, as much as XML has gained enormous momentum and widespread usage, relational database has long been the backbone infrastructure of enterprise applications; and proven to be effective and efficient. In the foreseeable future, relational database technology will remain the dominant technology, with major adoptions with respect to XML.

Many relational database vendors have provided extended database features to incorporate XML technology. For instance, SQL server has XQuery support and has a new XML data type. Likewise, middleware vendors have also provided native support for XML, like ADO.NET.

In ADO.NET, the central data object - the DataSet - represents a hierarchical, disconnected data cache. Its design is based on XML, which makes the translation between relational data and XML easy. The following discussion will use some examples to illustrate the ways to map a DataSet from and to a XML file.

Load a DataSet from XML

To load a DataSet from an XML file, simply call the DataSet.ReadXml method, like so:

Figure 1 shows a screenshot of the output:

Figure 1: Screenshot of the output

In this example, a flat XML file with no hierarchy or nested element is selected. As a result, the DataSet contains only one table and it can be conveniently bound to a GridView.

Loading Schema Information from XML

The ReadXmlSchema or the InferXmlSchema methods of the DataSet allow you to load DataSet schema information from an XML document. Alternatively, you may use the overloaded ReadXml method with XmlReadMode.InferSchema as the second argument.

DataSet Schema information can be written to an .xsd file using the WriteXmlSchema method:

The following is the input XML file, books2.xml:

And here's the output file, books2.xsd:

<?xml version="1.0" standalone="yes"?>
<xs:schema id="bookstore" xmlns="" xmlns:xs="http://www.w3.org/2001/XMLSchema"

xmlns:msdata="urn:schemas-microsoft-com:xml-msdata">
  <xs:element name="author">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="first-name" type="xs:string" minOccurs="0" />
        <xs:element name="last-name" type="xs:string" minOccurs="0" />
      </xs:sequence>
    </xs:complexType>
  </xs:element>
  <xs:element name="bookstore" msdata:IsDataSet="true" msdata:UseCurrentLocale="true">
    <xs:complexType>
      <xs:choice minOccurs="0" maxOccurs="unbounded">
        <xs:element ref="author" />
        <xs:element name="book">
          <xs:complexType>
            <xs:sequence>
              <xs:element name="title" type="xs:string" minOccurs="0" />
              <xs:element name="price" type="xs:string" minOccurs="0" />
              <xs:element ref="author" minOccurs="0" maxOccurs="unbounded" />
              <xs:element name="authors" minOccurs="0" maxOccurs="unbounded">
                <xs:complexType>
                  <xs:sequence>
                    <xs:element ref="author" minOccurs="0" maxOccurs="unbounded" />
                  </xs:sequence>
                </xs:complexType>
              </xs:element>
            </xs:sequence>
          </xs:complexType>
        </xs:element>
      </xs:choice>
    </xs:complexType>
  </xs:element>
</xs:schema>

The following figure shows a screenshot of the output table relationship:

Figure 2: Output table relationship

A somewhat more complicated XML file with a hierarchy of 3 tiers has been chosen for the example. It is worth noting how the relationship is constructed from the XML to DataSet transformation. The hierarchies are automatically translated into three tables and primary-foreign key pairs are inserted.

DataSet with a single table to XML

Want to map a DataSet containing a single table to XML? Piece of cake. The following code illustrates how to fork some information from an Access database into a table and write it to a XML file.

// DataSet to XML. 
    private void SingleTableToXml()
    {
        string connectionstr = @"Provider=Microsoft.Jet.OLEDB.4.0;Data Source=" + Server.MapPath(@"data\northwind.mdb");

OleDbConnection conn = new OleDbConnection(connectionstr);

DataSet myDS = new DataSet("DataSetTransformed");

string sql="SELECT CategoryID, CategoryName, Description FROM Categories";
        //Create a DataAdapter to load data from original data source to the DataSet
        OleDbDataAdapter myAdapter = new OleDbDataAdapter(sql, conn);

myAdapter.Fill(myDS, "Categories");

//disconnect the database
        conn.Close();

//Write out the schema that this DataSet created, 
        //use the WriteXmlSchema method of the DataSet class
        myDS.WriteXmlSchema(Server.MapPath("categories.xsd"));
          
 
        // To write out the contents of the DataSet as XML, 
        //use a file name to call the WriteXml method of the DataSet class
        myDS.WriteXml(Server.MapPath("categoies.xml"), XmlWriteMode.IgnoreSchema);
 
    }

Here's a segment of the output XML file:

As you can see, the root element of the XML file takes the name of the DataSet. In .NET 2.0, many of the tasks that could previously only be performed by a DataSet can also be tackled by a DataTable; such as XML transformation.

DataSet with two tables in master-detail relationship

It gets a little complicated and code-heavy to map to an appropriately nested XML from a DataSet with master-detail tables. First, you need to specify the primary key of the master table. Second, you have to register the foreign key of the detail table. Finally, you must set to true the Nested property of the DataSet.

// DataSet to XML. 
    private void MasterDetailTableToXml()
    {
        string connectionstr = @"Provider=Microsoft.Jet.OLEDB.4.0;Data Source=" + Server.MapPath(@"data\northwind.mdb");

OleDbConnection conn = new OleDbConnection(connectionstr);

DataSet myDS = new DataSet("DataSetTransformed");
        
        string sql = "SELECT CategoryID, CategoryName, Description FROM Categories";
        //Create a DataAdapter to load data from original data source to the DataSet
        OleDbDataAdapter myAdapter = new OleDbDataAdapter(sql, conn);
      
        myAdapter.Fill(myDS, "Categories");

sql = "SELECT ProductID, ProductName, CategoryID, UnitPrice FROM Products";
   
        //Load detail table from original data source to the DataSet
        myAdapter.SelectCommand = new OleDbCommand(sql, conn);
        myAdapter.Fill(myDS, "Products");

//disconnect the database
       conn.Close();

//Get the primary key column from the master table
       DataColumn primarykey = myDS.Tables["Categories"].Columns["CategoryID"];

//Get the foreign key column from the detail table
       DataColumn foreignkey = myDS.Tables["Products"].Columns["CategoryID"];

//Assign a relation
       DataRelation relation = myDS.Relations.Add(primarykey, foreignkey);

//Ask ADO.NET to generate nested XML nodes
       relation.Nested = true;

//Write out the schema that this DataSet created, 
       //use the WriteXmlSchema method of the DataSet class
       myDS.WriteXmlSchema(Server.MapPath("products.xsd"));

// To write out the contents of the DataSet as XML, 
       //use a file name to call the WriteXml method of the DataSet class
       myDS.WriteXml(Server.MapPath("products.xml"), XmlWriteMode.IgnoreSchema);
       
    }

The following is the output XML File:

DataSet with three or more tables to XML

While it is quite manageable and even fun to translate a two-master-detail-tabled DataSet to an XML file, it is quite hopeless to go beyond. I mean: If you want to rely only on the built-in methods of DataSet and get the XML file elegantly nested in multiple levels. Of course, it is a different story if you are willing to sweat and traverse among the rows and columns of the different tables of a DataSet.

Summary

While relational database remains the major database technology and crucial infrastructure of most business and web applications, XML has shown clear advantages and great promises in data exchange and storage. Therefore, many database and middleware vendors have taken strides to enable the integration and communication of both technologies. ADO .NET is no exception. And the above article showed how easy it is to transform from XML to DataSet and backwards.

References

About Xun Ding

Web developer, Data Analyst, GIS Programmer

This author has published 11 articles on DotNetSlackers. View other articles or the complete profile here.

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Discussion

Subject	Author	Date
LINQ to XSD?	Granville Barnett	9/11/2007 1:48 PM
Hi Xun, Have you looked at LINQ to XSD? The url is huge so I won't post it here but give it a search. Granville http://gbarnett.org Reply \| Permanent link
RE: LINQ to XSD?	Xun Ding	9/12/2007 1:44 PM
Thanks a lot for the suggestion. I know LINQ has become a major force working across database, xml and other types of data source. However, my knowledge of it is bare. So need to dig into it later. The online literature about LINQ is indeed numerous, that addes some headache. It is always a headache to me trying to catch the fast going technology train. Try, try, never stop ... Xun Beautiful Girls Programmatically Speaking ... Reply \| Permanent link
RE: RE: LINQ to XSD?	Sonu Kapoor	9/12/2007 1:49 PM
Xun, We do have a XLINQ series on DotNetSlackers, just in the case you would like to read more about it. It was written by Granville :) Here is the URL: http://dotnetslackers.com/articles/csharp/IntroducingLINQ1.aspx [MVP since 2005] [MCAD] Webmaster of DotNetSlackers Question or Suggestion? Feel free to ask my any .NET question Our Posting FAQ Reply \| Permanent link
RE: RE: RE: LINQ to XSD?	Xun Ding	9/12/2007 3:38 PM
Thanks. It amazes that you are so quick, resourceful, attentive, efficient taking care of all issues. Xun Beautiful Girls Programmatically Speaking ... Reply \| Permanent link
RE: RE: RE: RE: LINQ to XSD?	Sonu Kapoor	9/12/2007 8:44 PM
My pleasure. [MVP since 2005] [MCAD] Webmaster of DotNetSlackers Question or Suggestion? Feel free to ask my any .NET question Our Posting FAQ Reply \| Permanent link