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FOR XML Query Compared to Nested FOR XML Query

This topic compares a single-level FOR XML query to a nested FOR XML query. One of the benefits of using nested FOR XML queries is that you can specify a combination of attribute-centric and element-centric XML for query results. The example demonstrates this.

The following SELECT query retrieves product category and subcategory information in the AdventureWorks2008R2 database. There is no nested FOR XML in the query.

USE AdventureWorks2008R2;
GO
SELECT   ProductCategory.ProductCategoryID, 
         ProductCategory.Name as CategoryName,
         ProductSubcategory.ProductSubcategoryID, 
         ProductSubcategory.Name
FROM     Production.ProductCategory
INNER JOIN Production.ProductSubcategory
    ON ProductCategory.ProductCategoryID = ProductSubcategory.ProductCategoryID
ORDER BY ProductCategoryID
FOR XML AUTO, TYPE;
GO

This is the partial result:

<ProductCategory ProductCategoryID="1" CategoryName="Bike">

<ProductSubCategory ProductSubCategoryID="1" Name="Mountain Bike"/>

<ProductSubCategory ProductSubCategoryID="2" Name="Road Bike"/>

<ProductSubCategory ProductSubCategoryID="3" Name="Touring Bike"/>

</ProductCategory>

...

If you specify the ELEMENTS directive in the query, you receive an element-centric result, as shown in the following result fragment:

<ProductCategory>

<ProductCategoryID>1</ProductCategoryID>

<CategoryName>Bike</CategoryName>

<ProductSubCategory>

<ProductSubCategoryID>1</ProductSubCategoryID>

<Name>Mountain Bike</Name>

</ProductSubCategory>

<ProductSubCategory>

...

</ProductSubCategory>

</ProductCategory>

Next, assume that you want to generate an XML hierarchy that is a combination of attribute-centric and element-centric XML, as shown in the following fragment:

<ProductCategory ProductCategoryID="1" CategoryName="Bike">

<ProductSubCategory>

<ProductSubCategoryID>1</ProductSubCategoryID>

<SubCategoryName>Mountain Bike</SubCategoryName></ProductSubCategory>

<ProductSubCategory>

...

<ProductSubCategory>

...

</ProductCategory>

In the previous fragment, product category information such as category ID and category name are attributes. However, the subcategory information is element-centric. To construct the <ProductCategory> element, you can write a FOR XML query as shown in the following:

SELECT ProductCategoryID, Name as CategoryName
FROM Production.ProductCategory ProdCat
ORDER BY ProductCategoryID
FOR XML AUTO, TYPE;

This is the result:

< ProdCat ProductCategoryID="1" CategoryName="Bikes" />

< ProdCat ProductCategoryID="2" CategoryName="Components" />

< ProdCat ProductCategoryID="3" CategoryName="Clothing" />

< ProdCat ProductCategoryID="4" CategoryName="Accessories" />

To construct the nested <ProductSubCategory> elements in the XML you want, you then add a nested FOR XML query, as shown in the following:

SELECT ProductCategoryID, Name as CategoryName,
       (SELECT ProductSubcategoryID, Name SubcategoryName
        FROM   Production.ProductSubcategory
        WHERE ProductSubcategory.ProductCategoryID = 
              ProductCategory.ProductCategoryID
        FOR XML AUTO, TYPE, ELEMENTS
       )
FROM Production.ProductCategory
ORDER BY ProductCategoryID
FOR XML AUTO, TYPE;

Note the following in the previous query:

  • The inner FOR XML query retrieves product subcategory information. The ELEMENTS directive is added in the inner FOR XML to generate element-centric XML that is added to the XML generated by the outer query. By default, the outer query generates attribute-centric XML.

  • In the inner query, the TYPE directive is specified so the result is of xml type. If TYPE is not specified, the result is returned as nvarchar(max) type and the XML data is returned as entities.

  • The outer query also specifies the TYPE directive. Therefore, the result of this query is returned to the client as xml type.

This is the partial result:

<ProductCategory ProductCategoryID="1" CategoryName="Bike">

<ProductSubCategory>

<ProductSubCategoryID>1</ProductSubCategoryID>

<SubCategoryName>Mountain Bike</SubCategoryName></ProductSubCategory>

<ProductSubCategory>

...

<ProductSubCategory>

...

</ProductCategory>

The following query is just an extension of the previous query. It shows the full product hierarchy in the AdventureWorks2008R2 database. This includes the following:

  • Product categories

  • Product subcategories in each category

  • Product models in each subcategory

  • Products in each model

You might find the following query useful in understanding the AdventureWorks2008R2 database:

SELECT ProductCategoryID, Name as CategoryName,
       (SELECT ProductSubcategoryID, Name SubcategoryName,
               (SELECT ProductModel.ProductModelID, 
                       ProductModel.Name as ModelName,
                       (SELECT ProductID, Name as ProductName, Color
                        FROM   Production.Product
                        WHERE  Product.ProductModelID = 
                               ProductModel.ProductModelID
                        FOR XML AUTO, TYPE)
                FROM   (SELECT DISTINCT ProductModel.ProductModelID, 
                               ProductModel.Name
                        FROM   Production.ProductModel, 
                               Production.Product
                        WHERE  ProductModel.ProductModelID = 
                               Product.ProductModelID
                        AND    Product.ProductSubcategoryID = 
                               ProductSubcategory.ProductSubcategoryID) 
                                  ProductModel
                FOR XML AUTO, type
               )
        FROM Production.ProductSubcategory
        WHERE ProductSubcategory.ProductCategoryID = 
              ProductCategory.ProductCategoryID
        FOR XML AUTO, TYPE, ELEMENTS
       )
FROM Production.ProductCategory
ORDER BY ProductCategoryID
FOR XML AUTO, TYPE;

This is the partial result:

<Production.ProductCategory ProductCategoryID="1" CategoryName="Bikes">

<Production.ProductSubCategory>

<ProductSubCategoryID>1</ProductSubCategoryID>

<SubCategoryName>Mountain Bikes</SubCategoryName>

<ProductModel ProductModelID="19" ModelName="Mountain-100">

<Production.Product ProductID="771"

ProductName="Mountain-100 Silver, 38" Color="Silver" />

<Production.Product ProductID="772"

ProductName="Mountain-100 Silver, 42" Color="Silver" />

<Production.Product ProductID="773"

ProductName="Mountain-100 Silver, 44" Color="Silver" />

</ProductModel>

If you remove the ELEMENTS directive from the nested FOR XML query that generates product subcategories, the whole result is attribute-centric. You can then write this query without nesting. The addition of ELEMENTS results in an XML that is partly attribute-centric and partly element-centric. This result cannot be generated by a single-level, FOR XML query.

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