INTRODUCTION Essential ADO.NET 49.95 Like a SQL query,

INTRODUCTION

  
Essential ADO.NET
49.95

Like a SQL query, an XPath query simply produces a resultset consisting of possibly multiple instances of items; unlike in SQL, these results are not always rectangular in shape. XPath results can consist of nodesets of any shape or even scalar values. In SQL, database vendors can implement a variation of SQL-PSM (persistent stored modules) that composes possibly multiple SQL queries and some procedural code to produce a more complex result. SQL Server s variation of SQL-PSM is known as Transact- SQL. XML processing libraries implement an analogous concept by using an XML-based nonprocedural language called XSLT. Originally meant to produce nice looking HTML pages from XML input, XSLT has evolved into an almost full-fledged programming language. Vendors have even added proprietary extensions to XSLT to allow it to execute code routines in procedural programming languages like Visual Basic or C#. Since XPath and XSLT were not originally developed to process large amounts of data or data from multiple sources, a new programming language for XML, known as XQuery, has been developed. XQuery implements many of the best features of XPath and XSLT, and is developed from the ground up to allow queries that include multiple documents. It is also designed specifically to be optimizable. In addition, it adds some of the syntax features of SQL. XQuery s data can be strongly typed; this also assists in query optimization. XQuery includes a query language, the equivalent of SQL Server SELECT, but does not define a standard implementation of DML, SQLServer s INSERT, UPDATE, and DELETEstatements. SQL Server 2000 allowed users to define mapping schemas (normal XML schemas with extra annotations that mapped XML items and concepts to SQL items and concepts) that represented all or a portion of the database as a virtual XML document, and issue XPath queries against the resulting data structure. In addition, SQL Server 2000 extended Transact- SQL to enable relational resultsets to be returned as XML. This consists of support for a FOR XML clause; three different subcategories of FOR XML are supported. The SQL Server 2000 support allowed XML document composition from relational data and XML document decomposition into multiple relational tables; this will be discussed further in Chapter 8. SQL Server 2005 extends this support by adding direct support for XQuery. The XQuery engine runs directly inside SQL Server, as opposed to

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