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Working with the XML Data Type in SQL Server

By: Robert Sheldon
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The XML data type, introduced in SQL Server 2005, is a powerful construct. When used wisely, it can provide useful extensions to SQL Server. Robert Sheldon, in the first part of a series, describes how create and index a XML column in a table, and discusses when you should consider using an XML data type

Not all SQL Server data types are created equal. Just look at the XML data type. On the surface, it might seem like your run-of-the-mill type, except, of course, being geared toward XML data; but the ways in which it’s used, how its data is queried, and when and how XML columns should be indexed quickly sets the type apart from the rest of the crowd. And those differences are what matter when working within the extensible world of XML.

The XML data type, in fact, lies at the heart of understanding how to store and query XML data in a SQL Server database. That’s not to suggest that all XML data should be stored with the XML type, but knowing how the type works will help you determine when to use it and how to effectively access its data.

In some cases, you shouldn’t use the XML data type, but instead use large object storage—VARCHAR(MAX), NVARCHAR(MAX), or VARBINARY(MAX). For example, if you simply store your XML documents in the database and retrieve and update those documents as a whole—that is, if you never need to query or modify the individual XML components—you should consider using one of the large object data types. The same goes for XML files that you want to preserve in their original form, such as legal documents. If you need to retain an exact textual copy, use large object storage.

But the rest of the time, you should consider the XML data type. The type ensures that the data is well formed according to ISO standards, and it supports fine-grained queries and modifications to specific elements and attributes within the XML. You can also index an XML column and associate its data with an XML schema collection in order to preserve its content and structure. In addition, the XML data type lets you store data that follows a structure too fluid and complex to fit easily into a relational model.

However, when considering whether to use the XML data type, you should also be aware of its limitations. For instance, an XML column cannot be used as a key in an index, and a data value stored in an XML column cannot exceed 2 GB. You also cannot compare or sort data that uses the XML data type, nor can the data be used in a GROUP BY clause. For a complete description of the limitations on the XML data type, as well as other details about XML, see the topic “Implementing XML in SQL Server” in SQL Server Books Online.

Creating XML Database Objects
SQL Server lets you assign the XML data type to columns, variables, or parameters and store in any of these objects either XML documents or fragments. The data is considered a document if it has a single top-level element. Otherwise it falls under the category of fragment.

NOTE: You can also assign the XML data type to values returned by a function. However, function return values usually require XML components more complex than what we’ll cover in this article. For this reason, functions will be covered in a later article, after those XML components have been discussed.

When you assign the XML data type to a column, variable, or parameter, you can optionally associate an XML schema collection with the object, thus ensuring that data within that object conforms to schema specifications. In such cases, the object is referred to as typed. An XML object with no associated schema collection is considered untyped.

Creating Untyped XML Objects
An untyped XML object still requires that the data be well formed according to ISO standards; however, the data is not bound to an XML schema collection. You should choose untyped XML (the default) if you don’t have a schema to associate with the data or you don’t want to adhere to the constraints imposed by a schema. For instance, you might have a workable schema but might also need to store nonconforming fragments temporarily in the XML column.

To create an XML object in your database, you simply specify the XML data type as you would any other type. For instance, the following Transact-SQL code creates the Resumes tables, inserts data into the table, and then retrieves data from that table:
USE AdventureWorks2008R2;
DROP TABLE dbo.Resumes;
CREATE TABLE dbo.Resumes
CandidateResume XML
INSERT INTO Resumes (CandidateResume)
FROM HumanResources.JobCandidate;
SELECT * FROM Resumes;
Notice that the table includes the CandidateResume column, which is configured with the XML data type. Because this is an untyped column, you don’t have to specific any other parameters related to the XML. You can, of course, specify the nullability or other column properties, but XML is all you need to include to create an XML column.

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