Retrieve and Query XML Data
This topic describes the query options that you have to specify to query XML data. It also describes the parts of XML instances that are not preserved when they are stored in databases.
Features of an XML Instance That Are Not Preserved
SQL Server preserves the content of the XML instance, but does not preserve aspects of the XML instance that are not considered to be significant in the XML data model. This means that a retrieved XML instance might not be identical to the instance that was stored in the server, but will contain the same information.
The XML declaration in an instance is not preserved when the instance is stored in the database. For example:
CREATE TABLE T1 (Col1 int primary key, Col2 xml) GO INSERT INTO T1 values (1, '<?xml version="1.0" encoding="windows-1252" ?><doc></doc>') GO SELECT Col2 FROM T1
The result is
The XML declaration, such as
<?xml version='1.0'?>, is not preserved when storing XML data in an xml data type instance. This is by design. The XML declaration () and its attributes (version/encoding/stand-alone) are lost after data is converted to type xml. The XML declaration is treated as a directive to the XML parser. The XML data is stored internally as ucs-2. All other PIs in the XML instance are preserved.
Order of Attributes
The order of attributes in an XML instance is not preserved. When you query the XML instance stored in the xml type column, the order of attributes in the resulting XML may be different from the original XML instance.
Quotation Marks Around Attribute Values
Single quotation marks and double quotations marks around attribute values are not preserved. The attribute values are stored in the database as a name and value pair. The quotation marks are not stored. When an XQuery is executed against an XML instance, the resulting XML is serialized with double quotation marks around the attribute values.
DECLARE @x xml -- Use double quotation marks. SET @x = '<root a="1" />' SELECT @x GO DECLARE @x xml -- Use single quotation marks. SET @x = '<root a=''1'' />' SELECT @x GO
Both queries return =
<root a="1" />.
Namespace prefixes are not preserved. When you specify XQuery against an xml type column, the serialized XML result may return different namespace prefixes.
DECLARE @x xml SET @x = '<ns1:root xmlns:ns1="abc" xmlns:ns2="abc"> <ns2:SomeElement/> </ns1:root>' SELECT @x SELECT @x.query('/*') GO
The namespace prefix in the result may be different. For example:
Setting Required Query Options
When querying xml type columns or variables using xml data type methods, the following options must be set as shown.
|SET Options||Required Values|
If the options are not set as shown, queries and modifications on xml data type methods will fail.