xml Data Type Support in SQLXML 4.0
Beginning with SQL Server 2005, SQL Server supports XML typed data using the xml data type. This topic provides information about how SQLXML 4.0 recognizes instances of the xml data type and implements support for them.
Working with xml Data Types
To understand more about how to work with SQL tables that implement xml data type columns, the following examples are provided:
Task |
Example |
Topic |
|---|---|---|
How to map and include an xml column in an XML view |
"Mapping an XML element to an XML data type column" |
Default Mapping of XSD Elements and Attributes to Tables and Columns (SQLXML 4.0) |
How to insert data into an xml column with updategrams |
"Inserting data into an XML data type column" |
|
Bulk loading XML data into an xml column |
"Bulk loading in xml Data Type columns" |
Guidelines and Limitations
<xsd:any> cannot be mapped to a column including an xml data type. Support in SQLXML for this scenario is provided through the sql:overflow-field annotation. Another workaround is to map an xml data type field as an element of xsd:anyType. This workaround is demonstrated in the "Mapping an XML element to an XML data type column" example referenced in the table above.
XPath query into the contents of xml data type columns is not supported.
Using an xml data type column in annotations where it is not supported (such as sql:relationship and sql:key-fields) or allowed will result in SQL Server errors that will not be trapped by middle-tier components implementing SQLXML 4.0. This occurs because SQLXML does not require SQL type information. This is similar to the behavior of SQLXML for other data types, such as BLOB and binary types.
Mapping xml columns is supported only for XSD schemas. XDR schemas do not support mapping xml columns.
SQLXML 4.0 relies upon the XML parsing support provided in SQL Server. An xml column can be either mapped as typed XML or untyped XML. In either case, SQLXML 4.0 does not validate the input XML. If input XML is not valid or well-formed, SQL Server reports it to SQLXML, and propagates any relevant error information returned by the server to the user.
SQLXML 4.0 relies upon the limited support for DTDs provided in SQL Server. SQL Server allows for an internal DTD in xml data type data, which can be used to supply default values and to replace entity references with their expanded contents. SQLXML passes the XML data "as is" (including the internal DTD) to the server. You can convert DTDs to XML Schema (XSD) documents using third-party tools, and load the data with inline XSD schemas into the database.
SQLXML 4.0 does not preserve XML declaration processing instructions (for example, <?xml version='1.0'?>) based on the behavior of SQL Server. Instead, the XML declaration is treated as a directive to the SQL Server XML parser, and its attributes (version, encoding, and standalone) are lost after data is converted to the xml data type. The XML data is stored internally as UCS-2. All other processing instructions in the XML instance are preserved; they are allowed in the xml column and can be supported by SQLXML.