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title description author ms.author ms.reviewer ms.date ms.service ms.subservice ms.topic helpviewer_keywords
FOR XML query compared to nested FOR XML query
Learn about the benefits of nested FOR XML queries by viewing an example that compares a single-level FOR XML query to a nested FOR XML query.
MikeRayMSFT
mikeray
randolphwest
05/24/2024
sql
xml
conceptual
FOR XML query
queries [XML in SQL Server], comparing query types

FOR XML query compared to nested FOR XML query

[!INCLUDE SQL Server Azure SQL Database Azure SQL Managed Instance]

This article 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 benefit.

[!INCLUDE article-uses-adventureworks]

Examples

The following SELECT query retrieves product category and subcategory information in the [!INCLUDE ssSampleDBobject] database. There's no nested FOR XML in the query.

USE AdventureWorks2022;
GO

SELECT ProductCategory.ProductCategoryID,
    ProductCategory.Name AS CategoryName,
    ProductSubCategory.ProductSubCategoryID,
    ProductSubCategory.Name
FROM Production.ProductCategory,
    Production.ProductSubCategory
WHERE ProductCategory.ProductCategoryID = ProductSubCategory.ProductCategoryID
ORDER BY ProductCategoryID
FOR XML AUTO, TYPE;
GO

Here's 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;

Here's 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 code sample:

SELECT ProductCategoryID,
    Name AS CategoryName,
    (
        SELECT ProductSubCategoryID, Name AS 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 isn't 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.

Here's 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 [!INCLUDE ssSampleDBobject] database. This includes:

  • 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 [!INCLUDE ssSampleDBobject] database:

SELECT ProductCategoryID,
    Name AS CategoryName,
    (
        SELECT ProductSubCategoryID,
            Name AS 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;

Here's 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 can't be generated by a single-level, FOR XML query.

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