Lesson 2: Retrieving XML Data by Using SQL Server Server-Side Technologies in Visual Studio .NET

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Lesson 2: Retrieving XML Data by Using SQL Server Server-Side Technologies
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NULL as [Employee!2!id], NULL as [Employee!2!StartDate], NULL as [Employee!2!YearsInRole] FROM HumanResources.Department UNION ALL SELECT 2 as Tag, 1 as Parent, Department.[DepartmentID], NULL, History.EmployeeID, History.StartDate, DATEDIFF(year, History.[StartDate], GetDate()) FROM HumanResources.EmployeeDepartmentHistory History, HumanResources.Department WHERE Department.DepartmentID = History.DepartmentID AND History.EndDate IS NULL ORDER BY [Department!1!id], [Employee!2!YearsInRole] FOR XML EXPLICIT, ROOT('OldestEmployeeByDepartment')
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A partial result of executing this query is as follows:
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<OldestEmployeeByDepartment> <Department id="1" name="Engineering"> <Employee id="267" StartDate="2001-01-30T00:00:00" YearsInRole="5" /> <Employee id="270" StartDate="2001-02-18T00:00:00" YearsInRole="5" /> <Employee id="11" StartDate="1998-02-24T00:00:00" YearsInRole="8" /> <Employee id="12" StartDate="1998-03-03T00:00:00" YearsInRole="8" /> <Employee id="9" StartDate="1998-02-06T00:00:00" YearsInRole="8" /> <Employee id="3" StartDate="1997-12-12T00:00:00" YearsInRole="9" /> </Department> <Department id="2" name="Tool Design"> <Employee id="265" StartDate="2001-01-23T00:00:00" YearsInRole="5" /> <Employee id="263" StartDate="2001-01-05T00:00:00" YearsInRole="5" /> <Employee id="4" StartDate="2000-07-01T00:00:00" YearsInRole="6" /> <Employee id="5" StartDate="1998-01-11T00:00:00" YearsInRole="8" /> </Department> </OldestEmployeeByDepartment>
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If you execute the same query without the FOR XML EXPLICIT clause, you can see the Universal Table format, as Figure 8-2 shows.
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Managing XML Data
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Figure 8-2
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Universal Table format
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Here are some important observations to note about XML EXPLICIT formatting:
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You can combine multiple queries by a UNION operator to provide a complete Universal Table. There is one query for each level in the hierarchy. No root node is provided, so the XML structure is not a well-formed XML document. It represents an XML fragment unless you specify the ROOT instruction. Developers have full control over the number of levels that the XML structure will have. Column order in the SELECT clause is unimportant. Table aliases are ignored by the formatting mechanism.
Using FOR XML EXPLICIT
MORE INFO
For more information about the different settings available to FOR XML EXPLICIT, read the topic Using EXPLICIT Mode in SQL Server 2005 Books Online.
Quick Check
1. What FOR XML modes support mixing XML elements and attributes 2. What instruction is used to return well-formed XML documents instead of XML fragments when using FOR XML
Lesson 2: Retrieving XML Data by Using SQL Server Server-Side Technologies
Quick Check Answers
1. Only FOR XML PATH and FOR XML EXPLICIT allow you to choose independently the column formatting in an XML element or an XML attribute. 2. The ROOT( rootNodeName ) instruction allows you declare that the resulting XML must have a root element.
Retrieving XML Data from the XML Data Type
The XML data type provides greater searching and querying capabilities over an XML structure. More importantly, it provides developers with the ability to transform an XML instance into another XML instance; extract a value into the SQL type system; test for the existence of nodes and values inside the XML structure; and modify an existing XML structure by adding, updating, or deleting existing nodes. The XML data type provides five methods that enable you to manipulate the XML fragment. Table 8-3 lists these methods.
Table 8-3
XML Data Type Methods
Method query() value() exist()
Description Provides the ability to execute an XPATH or XQUERY expression and returns the resulting XML fragment. Provides the ability to execute an XPATH or XQUERY expression and returns a single scalar value that is converted into a SQL type. Provides the ability to execute an XPATH or XQUERY expression to check for the existence of nodes. If the query returns a node collection, the exist() method returns true; otherwise, the exist() method returns false. Provides XML data-manipulation capabilities (covered in later lessons). Provides the ability to execute an XPATH or XQUERY expression and returns the resulting XML fragment shredded into a row set (covered in later lessons).
modify() nodes()
You will see how to use the query(), value(), and exist() methods later in this lesson. And following lessons cover the use of the modify() and nodes() methods.
8
Managing XML Data
XQUERY and XPATH
The W3C has created two querying languages, XQUERY and XPATH, which combine to provide powerful capabilities for manipulating XML structures. XQUERY is a query language designed to query XML data. It has been designed around the following principles:
The result of an expression can be used as the input for another expression, much as in a functional programming language.
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