MSDN Tutorials

Working with XML Data in SQL Server

This tutorial explores the advanced features and best practices for handling XML data within SQL Server. Learn how to effectively store, query, and manipulate XML documents to leverage their structured and flexible nature in your database solutions.

Understanding XML Storage Options

SQL Server provides two primary data types for XML: XML (untyped) and XML(schema-collection) (typed). We'll delve into the differences, use cases, and performance implications of each.

Untyped XML

Untyped XML columns store XML data without validating it against an XML Schema Collection (XSD). This offers flexibility but sacrifices schema enforcement and certain query optimizations.

Typed XML

Typed XML columns are associated with an XML Schema Collection, allowing SQL Server to validate XML documents upon insertion or modification. This enables stronger type checking, better query performance, and richer IntelliSense.

Example of creating a table with typed XML:


CREATE XML SCHEMA COLLECTION MyBookSchema AS
N'
  
    
      
        
        
        
      
    
  
';
GO

CREATE TABLE Books (
    BookID INT PRIMARY KEY IDENTITY,
    BookData XML(MyBookSchema)
);
GO

Querying XML Data with XQuery

SQL Server leverages XQuery, a powerful query language for XML, to extract and manipulate data from XML columns. This includes using various XQuery functions and methods.

XQuery Methods

Key XQuery methods include:

  • .value(): Extracts a scalar value from an XML node.
  • .query(): Returns an XML fragment.
  • .exist(): Checks for the existence of a node.
  • .nodes(): Shreds XML into relational rows.

Example using .value() and .query():


INSERT INTO Books (BookData)
VALUES
(N'
  The Hitchhiker''s Guide to the Galaxy
  Douglas Adams
  1979
');
GO

-- Extracting the title
SELECT
    BookID,
    BookData.value('(/book/title)[1]', 'NVARCHAR(100)') AS Title
FROM Books;

-- Extracting the author and title as an XML fragment
SELECT
    BookID,
    BookData.query('(/book/author | /book/title)') AS AuthorAndTitle
FROM Books;

Shredding XML into Relational Rows

The .nodes() method is crucial for transforming XML data into a relational format, allowing you to join XML content with other tables or process it as rows.

Example using .nodes():


SELECT
    T.c.value('title[1]', 'NVARCHAR(100)') AS Title,
    T.c.value('author[1]', 'NVARCHAR(100)') AS Author,
    T.c.value('year[1]', 'INT') AS PublicationYear
FROM
    Books
CROSS APPLY
    BookData.nodes('/book') AS T(c);

Indexing XML Data

To improve the performance of queries against XML columns, SQL Server offers specialized XML indexes:

  • Primary XML Index: Indexes all nodes, values, and attributes within the XML instance.
  • Secondary XML Indexes (PATH, VALUE, PROPERTY): Provide targeted indexing for specific query patterns.

Example of creating a Primary XML Index:


CREATE PRIMARY XML INDEX IX_Books_BookData
ON Books(BookData);

Choosing the right index type is critical for optimizing query performance based on your access patterns.

Advanced Techniques and Considerations

  • XML Serialization: Learn how to generate XML from relational data using the FOR XML clause.
  • XML Validation Errors: Understand how to handle and diagnose validation errors when working with typed XML.
  • Performance Tuning: Explore strategies for optimizing XML query performance beyond indexing.
  • XML Namespaces: Effectively manage XML namespaces in your queries.