The Omedian Technical Architecture - Red Gate Software

Page 12 of 90Chapter 2: SQL Server XML Crib SheetXML Support in SQL ServerSQL Server is fundamentally a relational database, conforming where it can to the SQL standards. XML hasdifferent standards, so that integration is made more difficult by the fact that the XML data type standards arenot entirely the same as the relational data type standards. Mapping the two together is not alwaysstraightforward.XML has considerable attractions for the DBA or Database developer because it provides a way to pass a varietyof data structures as parameters, to store them, query and modify them. It also simplifies the process of providingbulk data-feeds. The challenge is to do this without increasing complexity or obscuring the clarity of the relationaldata-model.XML's major attraction for the programmer is that it can represent rowset (single table) and hierarchical(multiple-table) data, as well as relatively unstructured information such as text. This makes the creation,manipulation, and 'persisting' of objects far easier. XML can represent a complex Dataset consisting of severaltables that are related through primary and foreign keys, in such a way that it can be entirely reconstructed aftertransmission.XML documents can represent one or more typed rowsets (XML Information Set or 'Infoset'). To achieve this, areference to the relevant XML Schema should be contained in every XML document, or fragment, in order todata-type the XML content. SQL Server now provides a schema repository (library) for storing XML schemas,and it will use the appropriate schema to validate and store XML data.Loading XMLXML documents of any size are best loaded using the XML Bulk Load facility, which now has the ability to insertXML data from a flat file into an XML column. You can insert XML data from a file into base tables in SQLServer using the OPENROWSET table function, using the 'bulk rowset Provider', with an INSERT statement.The data can then be shredded to relational tables by using the xml.nodes function. (OpenXML can also be used.It is retained by SQL Server for compatibility with SQL Server 2000).Storing XMLXML documents, XML fragments and top-level text nodes can be stored as XML. XML can be used like anyother data type, as a table column, variable, parameter or function return-value. However, there are obviousrestrictions: although stored as UTF-16, the XML data is encoded and cannot be directly compared with otherXML data, neither can it be used as a primary or foreign key. It cannot have a unique constraint and the XMLdata is stored in a binary format rather than ASCII.Unlike other data types, the XML data type has its own methods to Create, Read, Update or Delete the elementswithin the XML document.XML data can have default values and can be checked by a variation of the RULE, where the validation isencapsulated within a user-defined function.XML data types can be allocated data by implicit conversion from the various CHAR formats, and TEXT, but noothers. There are no implicit conversions from XML data to other formats.Checking XML (XML Schemas)To specify the data type for an element or an attribute in an XML document, you use a schema. XML documentsare checked against XML Schemas. The XML Schema is a definition of the data structure used within an XMLDocument. This indicates, for example, whether a value such as "34.78" (which is stored as a text string withinthe XML) represents a character string, a currency value, or a numeric value.Page 12Chapter 2: SQL Server XML Crib Sheet