XQuery is based on the structure of XML and leverages this structure to provide query capabilities for the same range of data that XML stores. To be more precise, XQuery is defined in terms of the XQuery 1. Note that the data model used by XQuery is quite different from the classical relational model, which has no hierarchy, treats order as insignificant, and does not support identity.
Introduction to XQuery
XQuery is a functional language —instead of executing commands as procedural languages do, every query is an expression to be evaluated, and expressions can be combined quite flexibly with other expressions to create new expressions. This chapter gives a high-level introduction to the XQuery language by presenting a series of examples, each of which illustrates an important feature of the language and shows how it is used in practice. Some of the examples are drawn from [XQ-UC].
We cover most of the language features of XQuery, but also focus on teaching the idioms used to solve specific kinds of problems with XQuery. We start with a discussion of the structure of XML documents as input and output to queries and then present basic operations on XML-locating nodes in XML structures using path expressions , constructing XML structures with element constructors , and combining and restructuring information from XML documents using FLWOR expressions , sorting, conditional expressions, and quantified expressions.
After that, we explore operators and functions, discussing arithmetic operators, comparisons, some of the common functions in the XQuery function library, and how to write and call user-defined functions. Finally, we discuss how to import and use XML Schema types in queries.
Their behavior depends on the concept of node identity, which we'll cover in a later section. Strings, integers, decimals, and doubles are the only data types whose value can be entered into an XQuery program as literals. Constructor functions exist for all built-in types that convert strings or other values into values of its type:.
XML data appear in XQuery programs as nodes.
Nodes can be either created anew or selected from existing nodes using XPath expressions. Notice how Qexo's prompt changes to indicate the current expression nesting. Nodes have types.
A Light but Formal Introduction to XQuery | SpringerLink
Namespace nodes are handled through namespace declarations. The parentheses are part of the type name, not function calls. Nodes have identities. Two nodes have the same identity if and only if they are selected from the same spot in the same XML document.
Using Path Expressions To Match & Select Items
Newly constructed nodes always have a new identity. XQuery provides the doc and collection functions to bring external XML data into a program.
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The doc function takes a URI and returns a document node. The collection function interprets the URI in an implementation specific way. XQuery includes XPath 2. XPath expressions produce new node sequences out of old ones. Each step has an axis, a test and optional predicates. Each step works on the result of the previous steps and produces its own results for the next step.
A step goes through each node in the input sequence to generate partial results, which are then put together to form the output sequence. Here we selected the greetings element from the child axis of the XML document, the greeting elements from the descendant axis, greeting descendants whose from attribute has the value "weiqi" , the from attributes of greeting descendants, and the first greeting descendant. There is a lot more to XPath expressions that we cannot cover here. You can also select nodes by their types rather than names.
FLWOR, pronounced flower, stands for " for , let , where , order by , return ", after the five clauses of the expression. The for and let clauses introduce variables and bind them to values. The optional where clause filters the variables.
Learn XQuery in 10 Minutes: An XQuery Tutorial
The optional order by clause imposes an order on the variables. The return clause builds the result sequence. Notice that the use of return in XQuery is quite different from Java. It specifies the result of a sub-expression and does not imply returning from a function. The value of the expression is a sequence of three elements. Qexo's prompt changes to reflect the clauses we are in. Saxon puts an XML declaration in front of every document node or top level element in the sequence when they are printed. But Saxon's output format is highly configurable.
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The default empty item treatment is implementation-defined. Quantifier expressions test for a condition for all or some items in a sequence.