Using Nested Queries to Create Complex Hierarchical Structures in .NET framework

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Using Nested Queries to Create Complex Hierarchical Structures
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As you saw in previous examples, FOR XML RAW, AUTO, and PATH all provide different capabilities for creating complex hierarchical XML structures. XML RAW enables you to create one-level XML structures only. XML AUTO creates a new level per participating table, but the structure is repeated per row. XML PATH allows each column to specify its location in the XML structure, but again, the structure is repeated per row. By using nested queries, you can modify the XML structure so that a set of nodes can really be contained by a parent node; not for each row, but for a set of rows.
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Managing XML Data
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The following example retrieves the same information as the FOR XML RAW example shown previously in this lesson. The difference is that by using a nested query, we can create sublevels in the resulting XML structure:
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SELECT Department.[DepartmentID], Department.[Name], ( SELECT EmployeeDepartmentHistory.[EmployeeID] ,EmployeeDepartmentHistory.[StartDate] ,DATEDIFF(year, EmployeeDepartmentHistory.[StartDate], GetDate()) AS YearsToDate FROM HumanResources.EmployeeDepartmentHistory WHERE Department.DepartmentID = EmployeeDepartmentHistory.DepartmentID AND EmployeeDepartmentHistory.EndDate IS NULL ORDER BY EmployeeDepartmentHistory.[StartDate] FOR XML RAW('Employee'), TYPE ) AS Employees FROM HumanResources.Department ORDER BY Department.[DepartmentID] FOR XML RAW('Department'), ELEMENTS, ROOT ('OldestEmployeeByDepartment')
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A partial result of executing this query follows:
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<OldestEmployeeByDepartment> <Department> <DepartmentID>15</DepartmentID> <Name>Shipping and Receiving</Name> <Employees> <Employee EmployeeID="34" StartDate="1999-01-08T00:00:00" YearsToDate="7" /> <Employee EmployeeID="35" StartDate="1999-01-08T00:00:00" YearsToDate="7" /> <Employee EmployeeID="72" StartDate="1999-01-27T00:00:00" YearsToDate="7" /> <Employee EmployeeID="85" StartDate="1999-02-03T00:00:00" YearsToDate="7" /> <Employee EmployeeID="121" StartDate="1999-02-21T00:00:00" YearsToDate="7" /> <Employee EmployeeID="195" StartDate="1999-03-30T00:00:00" YearsToDate="7" /> </Employees> </Department> <Department> <DepartmentID>16</DepartmentID> <Name>Executive</Name> <Employees> <Employee EmployeeID="109" StartDate="1999-02-15T00:00:00" YearsToDate="7" /> <Employee EmployeeID="140" StartDate="2003-12-16T00:00:00" YearsToDate="3" /> </Employees> </Department> </OldestEmployeeByDepartment>
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Compare this XML structure with the previous structures shown for FOR XML RAW, FOR XML AUTO, and FOR XML PATH. This is a much more intuitive and rich structure.
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Lesson 2: Retrieving XML Data by Using SQL Server Server-Side Technologies
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Department information is formatted as element-centric, and Employee information is formatted as attribute-centric. The departments are under the root node, each contained in a parent <Department> node. The employees are nested together by department and contained in a parent <Employees> node. The ordering of the department information can be different from the ordering of the employee information.
Using TYPE in nested FOR XML queries
NOTE
Because you use the TYPE instruction in the FOR XML clause in nested queries, SQL Server interprets and manipulates the resulting XML as an XML type instead of simply copying it as text in the containing node.
Using FOR XML EXPLICIT
The formatting mode FOR XML EXPLICIT provides the greater degree of control for developers to be able to generate complex XML structures. For FOR XML EXPLICIT to work, the query result set must follow a specific pattern called a Universal Table. The Universal Table requires specific columns that must be provided, and columns aliases must be formatted using a specific pattern. This formatting provides metadata for the XML formatter in SQL Server 2005 to construct the XML, as Table 8-2 describes.
Table 8-2
FOR XML EXPLICIT Result Set Requirements
Option Tag column
Description Must be the first column in the result set. The Tag column indicates the depth in the XML structure, starting from 1. Must be the second column in the result set. The Parent column indicates the node parent in the XML structure. The node parent is identified by its Tag identifier.
Parent column
8
Managing XML Data
Table 8-2
FOR XML EXPLICIT Result Set Requirements
Option Column name pattern: ElementName!TagNumber!AttributeName! Directive
Description Data columns must provide an alias following this pattern. ElementName is the name you want to assign to the XML element. TagNumber indicates the level (according to the tag column) at which this node must be located. AttributeName is optional if you indicate a directive; it indicates the name to provide to the XML attribute that holds the value. Directive is optional; it provides more information to the XML formatting mechanism. Some of its possible values include these:
hide: Indicates that this column should not be included in the resulting XML structure. Use this value for columns you might need just for ordering purposes. element: Generate the column value as an XML element, not as an XML attribute. NULL values will be ignored. elementxsinil: Generate the column value as an XML element, not as an XML attribute. NULL values will not be ignored; an empty element will be provided. cdata: Generate the column value as an XML comment inside a CDATA section.
The Universal Table also requires specific ordering for the rows in the result set. The XML structure is constructed following row order; the rows in the result set must be ordered so that each parent node is immediately followed by its child nodes.
SELECT 1 as Tag, NULL as Parent, Department.[DepartmentID] as [Department!1!id], Department.[Name] as [Department!1!name],
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