The Omedian Technical Architecture - Red Gate Software

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Page 13 of 90Chapter 2: SQL Server XML Crib SheetIf, for example, the XML document represents an invoice, the XML Schema describes the relationship betweenthe elements and attributes, and specifies the data types for that invoice.You can check, or validate, untyped XML, whether used in a column, variable or parameter, by associating it withan XML Schema. Once checked, it becomes 'typed'. This ensures that the data types of the elements andattributes of the XML instance are contained, and defined, in the schema. These names are valid within theparticular 'namespace' specified. An XML Schema definition is itself an XML document. These are catalogued inSQL Server as XML Schema collections, and shredded in order to optimise Schema validation. They are tied tospecific SQL Schema within a database.Using typed XML introduces integrity checking and helps the performance of XQuery.Accessing Data in XMLXML Data type columns can be indexed, and manipulated, using XQuery and XML Data Manipulation Language(XML DML), which adds 'Insert', 'delete' and 'replace' to XQuery.To make data-access more effective, XML in SQL Server can be indexed. To be indexed, the XML must be acolumn in a table that already has a primary key. The index can be over the document structure, or for the valuesof the elements.The XML data type can be viewed or modified by a number of methods. One can determine whether a nodeexists, get its value, retrieve it as table-result of a query, or modify its value.XML can be read by the XML parser into a 'Document Object Model' (DOM, see below) and then accessedprogrammatically via methods and properties, but it is not really a suitable server-side technology, due to theoverhead of parsing the document into the model.Shredding XMLThe process of converting XML data into a format that can be used by a relational database is called 'Shredding",or decomposition. One can either use the NODES method on an XML data type or, from a Document ObjectModel (DOM), use the OpenXML function. OpenXML is retained in SQL 2005, but the NODES method isgenerally preferable because of its simplicity and performance.Converting relational data to XMLXML fragments, or documents, can be produced from SQL Queries against relational tables, using the SELECT .. . For XML syntax. An inline XSD Format schema can be produced, and added to the beginning of thedocument. This is convenient but not covered by a W3C standard.Converting XML to other formatsXML documents can be converted into other XML documents, or into formats such as HTML, using XSLStylesheets (see below). These are themselves XML documents that provide a mixture of commands and text. Itis applied to an XML document by processing it via a parser.Querying XML DocumentsXQueryXQuery, derived in part from SQL, is the dominant standard for querying XML data. It is a declarative,functional query language that operates on instances of the XQuery/XPath Data Model (XDM) to query yourXML, using a "tree-like" logical representation of the XML. With XQuery you can run queries against variablesand columns of the XML data type using the latter's associated methods.Page 13Chapter 2: SQL Server XML Crib Sheet
Page 14 of 90Chapter 2: SQL Server XML Crib SheetXQuery has been around for a while. It evolved from an XML query language called Quilt, which in turn wasderived from XML Path Language (XPath) version 1.0, SQL, and XQL.XQuery has similarities with SQL, but is by no means the same. SQL is a more complete language. The SELECTstatement is similar to XQuery's language, but XQuery has to deal with a more complex data model.The XQuery specification currently contains syntax and semantics for querying, but not for modifying XMLdocuments, though these are effected by extensions to XQuery, collectively called the XML Data ManipulationLanguage (XML DML). This allows you to modify the contents of the XML document. With XML DML onecan insert child or sibling nodes into a document, delete one or more nodes, or replace values in nodes.Microsoft thoughtfully provided extensions that allow T-SQL variables and columns to be used to bind relationaldata inside XML data. Server 2005 adds three keywords: insert, update, and delete. Each of these is used withinthe modify() method of the XML data type.The XDM that XQuery uses is unlike the Document Object Model (DOM). Each branch (or "node") of theXDM tree maintains a set of attributes describing the node. In the tree, each node has an XML node type, XDMdata type information, node content (string and typed representations), parent/child information, and possiblysome other information specific to the node type.FLWORXQuery's FLWOR expressions (For, Let, Where, Order by, and Return) iterates XML nodes using the 'for'clause, limits the results using the 'where' clause, sorts the results using the 'order by' clause, and returns theresults via the 'return' clause. These constructs greatly extend the versatility of XQuery, making it comparable toSQLXPathXPath was designed to navigate an XML document to retrieve the document's elements and attributes. It alsoprovides basic facilities for manipulation of strings, numbers and Booleans. It represents the document as a treeof nodes, and allows reference to nodes by absolute or relative paths. One can specify criteria for the nodes thatare returned in square brackets.XML Template QueriesAn XML template query is an XML document with one or more TSQL or XPath queries embedded in it,allowing you to query an XML document. The results can be transformed with an XSLT stylesheet. Templatequeries are used in client code to update SQL Server data. They are templates with attributes and elements thatspecify data requiring updating and how that is to be done.UpdateGramAn UpdateGram is an XML template that is used to insert, update or delete data in a database. It contains animage of the data before and after the required modification. It is usually transmitted to the server by a clientapplication. Each element usually represents one record in a table. The data is 'mapped' either implicitly orexplicitly. One can pass parameters to them.DiffGramThis is an XML document format that is used to synchronise offline changes in data with a database server. It isvery similar to an UpdateGram, but is less complex. It is generally used for 'persisting' the data in data objects.Page 14Chapter 2: SQL Server XML Crib Sheet
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