The static context of an expression is the information that is available during static analysis of the expression, prior to its evaluation. This information can be used to decide whether the expression contains a static error. If analysis of an expression relies on some component of the static context that has not been assigned a value, a static error is raised .

The individual components of the static context are summarized below. Rules governing the scope and initialization of these components can be found in .

The dynamic context of an expression is defined as information that is available at the time the expression is evaluated. If evaluation of an expression relies on some part of the dynamic context that has not been assigned a value, a dynamic error is raised .

The individual components of the dynamic context are summarized below. Further rules governing the semantics of these components can be found in .

The dynamic context consists of all the components of the static context, and the additional components listed below.

The first three components of the dynamic context (context item, context position, and context size) are called the focus of the expression. The focus enables the processor to keep track of which items are being processed by the expression.

Certain language constructs, notably the path expression E1/E2 and the predicate E1[E2], create a new focus for the evaluation of a sub-expression. In these constructs, E2 is evaluated once for each item in the sequence that results from evaluating E1. Each time E2 is evaluated, it is evaluated with a different focus. The focus for evaluating E2 is referred to below as the inner focus, while the focus for evaluating E1 is referred to as the outer focus. The inner focus exists only while E2 is being evaluated. When this evaluation is complete, evaluation of the containing expression continues with its original focus unchanged.

As described in , XQuery 1.1 defines a static analysis phase, which does not depend on input data, and a dynamic evaluation phase, which does depend on input data. Errors may be raised during each phase.

A static error is an error that must be detected during the static analysis phase. A syntax error is an example of a static error.

A dynamic error is an error that must be detected during the dynamic evaluation phase and may be detected during the static analysis phase. Numeric overflow is an example of a dynamic error.

A type error may be raised during the static analysis phase or the dynamic evaluation phase. During the static analysis phase, a type error occurs when the static type of an expression does not match the expected type of the context in which the expression occurs. During the dynamic evaluation phase, a type error occurs when the dynamic type of a value does not match the expected type of the context in which the value occurs.

The outcome of the static analysis phase is either success or one or more type errors, static errors, or statically-detected dynamic errors. The result of the dynamic evaluation phase is either a result value, a type error, or a dynamic error.

If more than one error is present, or if an error condition comes within the scope of more than one error defined in this specification, then any non-empty subset of these errors may be reported.

During the static analysis phase, if the Static Typing Feature is in effect and the static type assigned to an expression other than () or data(()) is empty-sequence(), a static error is raised . This catches cases in which a query refers to an element or attribute that is not present in the in-scope schema definitions, possibly because of a spelling error.

Independently of whether the Static Typing Feature is in effect, if an implementation can determine during the static analysis phase that an expression, if evaluated, would necessarily raise a type error or a dynamic error, the implementation may (but is not required to) report that error during the static analysis phase. However, the fn:error() function must not be evaluated during the static analysis phase.

In addition to static errors, dynamic errors, and type errors, an XQuery 1.1 implementation may raise warnings, either during the static analysis phase or the dynamic evaluation phase. The circumstances in which warnings are raised, and the ways in which warnings are handled, are implementation-defined.

In addition to the errors defined in this specification, an implementation may raise a dynamic error for a reason beyond the scope of this specification. For example, limitations may exist on the maximum numbers or sizes of various objects. Any such limitations, and the consequences of exceeding them, are implementation-dependent.

The errors defined in this specification are identified by QNames that have the form err:XXYYnnnn, where:

The method by which an XQuery 1.1 processor reports error information to the external environment is implementation-defined.

An error can be represented by a URI reference that is derived from the error QName as follows: an error with namespace URI NS and local part LP can be represented as the URI reference NS # LP . For example, an error whose QName is err:XPST0017 could be represented as http://www.w3.org/2005/xqt-errors#XPST0017.

Except as noted in this document, if any operand of an expression raises a dynamic error, the expression also raises a dynamic error. If an expression can validly return a value or raise a dynamic error, the implementation may choose to return the value or raise the dynamic error. For example, the logical expression expr1 and expr2 may return the value false if either operand returns false, or may raise a dynamic error if either operand raises a dynamic error.

If more than one operand of an expression raises an error, the implementation may choose which error is raised by the expression. For example, in this expression:

both the sub-expressions ($x div $y) and xs:decimal($z) may raise an error. The implementation may choose which error is raised by the "+" expression. Once one operand raises an error, the implementation is not required, but is permitted, to evaluate any other operands.

In addition to its identifying QName, a dynamic error may also carry a descriptive string and one or more additional values called error values. An implementation may provide a mechanism whereby an application-defined error handler can process error values and produce diagnostic messages.

A dynamic error may be raised by a built-in function or operator. For example, the div operator raises an error if its operands are xs:decimal values and its second operand is equal to zero. Errors raised by built-in functions and operators are defined in .

A dynamic error can also be raised explicitly by calling the fn:error function, which only raises an error and never returns a value. This function is defined in . For example, the following function call raises a dynamic error, providing a QName that identifies the error, a descriptive string, and a diagnostic value (assuming that the prefix app is bound to a namespace containing application-defined error codes):

Because different implementations may choose to evaluate or optimize an expression in different ways, certain aspects of the detection and reporting of dynamic errors are implementation-dependent, as described in this section.

An implementation is always free to evaluate the operands of an operator in any order.

In some cases, a processor can determine the result of an expression without accessing all the data that would be implied by the formal expression semantics. For example, the formal description of filter expressions suggests that $s[1] should be evaluated by examining all the items in sequence $s, and selecting all those that satisfy the predicate position()=1. In practice, many implementations will recognize that they can evaluate this expression by taking the first item in the sequence and then exiting. If $s is defined by an expression such as //book[author eq 'Berners-Lee'], then this strategy may avoid a complete scan of a large document and may therefore greatly improve performance. However, a consequence of this strategy is that a dynamic error or type error that would be detected if the expression semantics were followed literally might not be detected at all if the evaluation exits early. In this example, such an error might occur if there is a book element in the input data with more than one author subelement.

The extent to which a processor may optimize its access to data, at the cost of not detecting errors, is defined by the following rules.

Consider an expression Q that has an operand (sub-expression) E. In general the value of E is a sequence. At an intermediate stage during evaluation of the sequence, some of its items will be known and others will be unknown. If, at such an intermediate stage of evaluation, a processor is able to establish that there are only two possible outcomes of evaluating Q, namely the value V or an error, then the processor may deliver the result V without evaluating further items in the operand E. For this purpose, two values are considered to represent the same outcome if their items are pairwise the same, where nodes are the same if they have the same identity, and values are the same if they are equal and have exactly the same type.

There is an exception to this rule: If a processor evaluates an operand E (wholly or in part), then it is required to establish that the actual value of the operand E does not violate any constraints on its cardinality. For example, the expression $e eq 0 results in a type error if the value of $e contains two or more items. A processor is not allowed to decide, after evaluating the first item in the value of $e and finding it equal to zero, that the only possible outcomes are the value true or a type error caused by the cardinality violation. It must establish that the value of $e contains no more than one item.

These rules apply to all the operands of an expression considered in combination: thus if an expression has two operands E1 and E2, it may be evaluated using any samples of the respective sequences that satisfy the above rules.

The rules cascade: if A is an operand of B and B is an operand of C, then the processor needs to evaluate only a sufficient sample of B to determine the value of C, and needs to evaluate only a sufficient sample of A to determine this sample of B.

The effect of these rules is that the processor is free to stop examining further items in a sequence as soon as it can establish that further items would not affect the result except possibly by causing an error. For example, the processor may return true as the result of the expression S1 = S2 as soon as it finds a pair of equal values from the two sequences.

Another consequence of these rules is that where none of the items in a sequence contributes to the result of an expression, the processor is not obliged to evaluate any part of the sequence. Again, however, the processor cannot dispense with a required cardinality check: if an empty sequence is not permitted in the relevant context, then the processor must ensure that the operand is not an empty sequence.

Examples:

For a variety of reasons, including optimization, implementations may rewrite expressions into a different form. There are a number of rules that limit the extent of this freedom:

Expression rewrite is illustrated by the following examples.

An ordering called document order is defined among all the nodes accessible during processing of a given query, which may consist of one or more trees (documents or fragments). Document order is defined in , and its definition is repeated here for convenience. The node ordering that is the reverse of document order is called reverse document order.

Document order is a total ordering, although the relative order of some nodes is implementation-dependent. Informally, document order is the order in which nodes appear in the XML serialization of a document. Document order is stable, which means that the relative order of two nodes will not change during the processing of a given query, even if this order is implementation-dependent.

Within a tree, document order satisfies the following constraints:

The relative order of nodes in distinct trees is stable but implementation-dependent, subject to the following constraint: If any node in a given tree T1 is before any node in a different tree T2, then all nodes in tree T1 are before all nodes in tree T2.

The semantics of some XQuery 1.1 operators depend on a process called atomization. Atomization is applied to a value when the value is used in a context in which a sequence of atomic values is required. The result of atomization is either a sequence of atomic values or a type error [err:FOTY0012]. Atomization of a sequence is defined as the result of invoking the fn:data function on the sequence, as defined in .

The semantics of fn:data are repeated here for convenience. The result of fn:data is the sequence of atomic values produced by applying the following rules to each item in the input sequence:

Atomization is used in processing the following types of expressions:

Under certain circumstances (listed below), it is necessary to find the effective boolean value of a value. The effective boolean value of a value is defined as the result of applying the fn:boolean function to the value, as defined in .

The dynamic semantics of fn:boolean are repeated here for convenience:

The effective boolean value of a sequence is computed implicitly during processing of the following types of expressions:

XQuery 1.1 has a set of functions that provide access to input data. These functions are of particular importance because they provide a way in which an expression can reference a document or a collection of documents. The input functions are described informally here; they are defined in .

An expression can access input data either by calling one of the input functions or by referencing some part of the dynamic context that is initialized by the external environment, such as a variable or context item.

The input functions supported by XQuery 1.1 are as follows:

In certain places in the XQuery grammar, a statically known valid URI is required. These places are denoted by the grammatical symbol URILiteral. For example, URILiterals are used to specify namespaces and collations, both of which must be statically known.

Syntactically, a URILiteral is identical to a StringLiteral: a sequence of zero or more characters enclosed in single or double quotes. However, an implementation MAY raise a static error if the value of a URILiteral is of nonzero length and is not in the lexical space of xs:anyURI.

As in a string literal, any predefined entity reference (such as &), character reference (such as •), or EscapeQuot or EscapeApos (for example, "") is replaced by its appropriate expansion. Certain characters, notably the ampersand, can only be represented using a predefined entity reference or a character reference.

The URILiteral is subjected to whitespace normalization as defined for the xs:anyURI type in : this means that leading and trailing whitespace is removed, and any other sequence of whitespace characters is replaced by a single space (#x20) character. Whitespace normalization is done after the expansion of character references, so writing a newline (for example) as 
 does not prevent its being normalized to a space character.

The URILiteral is not automatically subjected to percent-encoding or decoding as defined in . Any process that attempts to resolve the URI against a base URI, or to dereference the URI, may however apply percent-encoding or decoding as defined in the relevant RFCs.

The following is an example of a valid URILiteral:

The in-scope schema types in the static context are initialized with certain predefined schema types, including the built-in schema types in the namespace http://www.w3.org/2001/XMLSchema, which has the predefined namespace prefix xs. The schema types in this namespace are defined in and augmented by additional types defined in . Element and attribute declarations in the xs namespace are not implicitly included in the static context. The schema types defined in are summarized below.

The relationships among the schema types in the xs namespace are illustrated in Figure 2. A more complete description of the XQuery 1.1 type hierarchy can be found in .

Every node has a typed value and a string value. The typed value of a node is a sequence of atomic values and can be extracted by applying the fn:data function to the node. The string value of a node is a string and can be extracted by applying the fn:string function to the node. Definitions of fn:data and fn:string can be found in .

An implementation may store both the typed value and the string value of a node, or it may store only one of these and derive the other as needed. The string value of a node must be a valid lexical representation of the typed value of the node, but the node is not required to preserve the string representation from the original source document. For example, if the typed value of a node is the xs:integer value 30, its string value might be "30" or "0030".

The typed value, string value, and type annotation of a node are closely related, and are defined by rules found in the following locations:

As a convenience to the reader, the relationship between typed value and string value for various kinds of nodes is summarized and illustrated by examples below.

Whenever it is necessary to refer to a type in an XQuery 1.1 expression, the SequenceType syntax is used.

With the exception of the special type empty-sequence(), a sequence type consists of an item type that constrains the type of each item in the sequence, and a cardinality that constrains the number of items in the sequence. Apart from the item type item(), which permits any kind of item, item types divide into node types (such as element()) and atomic types (such as xs:integer).

Item types representing element and attribute nodes may specify the required type annotations of those nodes, in the form of a schema type. Thus the item type element(*, us:address) denotes any element node whose type annotation is (or is derived from) the schema type named us:address.

Here are some examples of sequence types that might be used in XQuery 1.1 expressions:

During evaluation of an expression, it is sometimes necessary to determine whether a value with a known dynamic type "matches" an expected sequence type. This process is known as SequenceType matching. For example, an instance of expression returns true if the dynamic type of a given value matches a given sequence type, or false if it does not.

QNames appearing in a sequence type have their prefixes expanded to namespace URIs by means of the statically known namespaces and (where applicable) the default element/type namespace. An unprefixed attribute QName is in no namespace. Equality of QNames is defined by the eq operator.

The rules for SequenceType matching compare the dynamic type of a value with an expected sequence type. These rules are a subset of the formal rules that match a value with an expected type defined in , because the Formal Semantics must be able to match values against types that are not expressible using the SequenceType syntax.

Some of the rules for SequenceType matching require determining whether a given schema type is the same as or derived from an expected schema type. The given schema type may be "known" (defined in the in-scope schema definitions), or "unknown" (not defined in the in-scope schema definitions). An unknown schema type might be encountered, for example, if a source document has been validated using a schema that was not imported into the static context. In this case, an implementation is allowed (but is not required) to provide an implementation-dependent mechanism for determining whether the unknown schema type is derived from the expected schema type. For example, an implementation might maintain a data dictionary containing information about type hierarchies.

The use of a value whose dynamic type is derived from an expected type is known as subtype substitution. Subtype substitution does not change the actual type of a value. For example, if an xs:integer value is used where an xs:decimal value is expected, the value retains its type as xs:integer.

The definition of SequenceType matching relies on a pseudo-function named derives-from( AT, ET ), which takes an actual simple or complex schema type AT and an expected simple or complex schema type ET, and either returns a boolean value or raises a type error . The pseudo-function derives-from is defined below and is defined formally in .

The rules for SequenceType matching are given below, with examples (the examples are for purposes of illustration, and do not cover all possible cases).

A literal is a direct syntactic representation of an atomic value. XQuery 1.1 supports two kinds of literals: numeric literals and string literals.

The value of a numeric literal containing no "." and no e or E character is an atomic value of type xs:integer. The value of a numeric literal containing "." but no e or E character is an atomic value of type xs:decimal. The value of a numeric literal containing an e or E character is an atomic value of type xs:double. The value of the numeric literal is determined by casting it to the appropriate type according to the rules for casting from xs:untypedAtomic to a numeric type as specified in .

The value of a string literal is an atomic value whose type is xs:string and whose value is the string denoted by the characters between the delimiting apostrophes or quotation marks. If the literal is delimited by apostrophes, two adjacent apostrophes within the literal are interpreted as a single apostrophe. Similarly, if the literal is delimited by quotation marks, two adjacent quotation marks within the literal are interpreted as one quotation mark.

A string literal may contain a predefined entity reference. A predefined entity reference is a short sequence of characters, beginning with an ampersand, that represents a single character that might otherwise have syntactic significance. Each predefined entity reference is replaced by the character it represents when the string literal is processed. The predefined entity references recognized by XQuery are as follows:

A string literal may also contain a character reference. A character reference is an XML-style reference to a character, identified by its decimal or hexadecimal code point. For example, the Euro symbol (€) can be represented by the character reference €. Character references are normatively defined in Section 4.1 of the XML specification (it is implementation-defined whether the rules in or apply.) A static error is raised if a character reference does not identify a valid character in the version of XML that is in use.

Here are some examples of literal expressions:

The xs:boolean values true and false can be represented by calls to the built-in functions fn:true() and fn:false(), respectively.

Values of other atomic types can be constructed by calling the constructor function for the given type. The constructor functions for XML Schema built-in types are defined in . In general, the name of a constructor function for a given type is the same as the name of the type (including its namespace). For example:

Constructor functions can also be used to create special values that have no literal representation, as in the following examples:

xs:float("NaN") returns the special floating-point value, "Not a Number."

It is also possible to construct values of various types by using a cast expression. For example:

A variable reference is a QName preceded by a $-sign. Two variable references are equivalent if their local names are the same and their namespace prefixes are bound to the same namespace URI in the statically known namespaces. An unprefixed variable reference is in no namespace.

Every variable reference must match a name in the in-scope variables, which include variables from the following sources:

A variable may be declared in a Prolog, in the current module or an imported module. See for a discussion of modules and Prologs.

Every variable binding has a static scope. The scope defines where references to the variable can validly occur. It is a static error to reference a variable that is not in scope. If a variable is bound in the static context for an expression, that variable is in scope for the entire expression.

A reference to a variable that was declared external, but was not bound to a value by the external environment, raises a dynamic error .

If a variable reference matches two or more variable bindings that are in scope, then the reference is taken as referring to the inner binding, that is, the one whose scope is smaller. At evaluation time, the value of a variable reference is the value of the expression to which the relevant variable is bound. The scope of a variable binding is defined separately for each kind of expression that can bind variables.

Parentheses may be used to enforce a particular evaluation order in expressions that contain multiple operators. For example, the expression (2 + 4) * 5 evaluates to thirty, since the parenthesized expression (2 + 4) is evaluated first and its result is multiplied by five. Without parentheses, the expression 2 + 4 * 5 evaluates to twenty-two, because the multiplication operator has higher precedence than the addition operator.

Empty parentheses are used to denote an empty sequence, as described in .

A context item expression evaluates to the context item, which may be either a node (as in the expression fn:doc("bib.xml")/books/book[fn:count(./author)>1]) or an atomic value (as in the expression (1 to 100)[. mod 5 eq 0]).

If the context item is undefined, a context item expression raises a dynamic error .

The built-in functions supported by XQuery 1.1 are defined in . Additional functions may be declared in a Prolog, imported from a library module, or provided by the external environment as part of the static context.

A function call consists of a QName followed by a parenthesized list of zero or more expressions, called arguments. If the QName in the function call has no namespace prefix, it is considered to be in the default function namespace.

If the expanded QName and number of arguments in a function call do not match the name and arity of a function signature in the static context, a static error is raised .

A function call is evaluated as follows:

The function conversion rules are used to convert an argument value or a return value to its expected type; that is, to the declared type of the function parameter or return. The expected type is expressed as a sequence type. The function conversion rules are applied to a given value as follows:

Since the arguments of a function call are separated by commas, any argument expression that contains a top-level comma operator must be enclosed in parentheses. Here are some illustrative examples of function calls:

A step is a part of a path expression that generates a sequence of items and then filters the sequence by zero or more predicates. The value of the step consists of those items that satisfy the predicates, working from left to right. A step may be either an axis step or a filter expression. Filter expressions are described in .

An axis step returns a sequence of nodes that are reachable from the context node via a specified axis. Such a step has two parts: an axis, which defines the "direction of movement" for the step, and a node test, which selects nodes based on their kind, name, and/or type annotation. If the context item is a node, an axis step returns a sequence of zero or more nodes; otherwise, a type error is raised . If ordering mode is ordered, the resulting node sequence is returned in document order; otherwise it is returned in implementation-dependent order. An axis step may be either a forward step or a reverse step, followed by zero or more predicates.

In the abbreviated syntax for a step, the axis can be omitted and other shorthand notations can be used as described in .

The unabbreviated syntax for an axis step consists of the axis name and node test separated by a double colon. The result of the step consists of the nodes reachable from the context node via the specified axis that have the node kind, name, and/or type annotation specified by the node test. For example, the step child::para selects the para element children of the context node: child is the name of the axis, and para is the name of the element nodes to be selected on this axis. The available axes are described in . The available node tests are described in . Examples of steps are provided in and .

A predicate consists of an expression, called a predicate expression, enclosed in square brackets. A predicate serves to filter a sequence, retaining some items and discarding others. In the case of multiple adjacent predicates, the predicates are applied from left to right, and the result of applying each predicate serves as the input sequence for the following predicate.

For each item in the input sequence, the predicate expression is evaluated using an inner focus, defined as follows: The context item is the item currently being tested against the predicate. The context size is the number of items in the input sequence. The context position is the position of the context item within the input sequence. For the purpose of evaluating the context position within a predicate, the input sequence is considered to be sorted as follows: into document order if the predicate is in a forward-axis step, into reverse document order if the predicate is in a reverse-axis step, or in its original order if the predicate is not in a step.

For each item in the input sequence, the result of the predicate expression is coerced to an xs:boolean value, called the predicate truth value, as described below. Those items for which the predicate truth value is true are retained, and those for which the predicate truth value is false are discarded.

The predicate truth value is derived by applying the following rules, in order:

Here are some examples of axis steps that contain predicates:

This section provides a number of examples of path expressions in which the axis is explicitly specified in each step. The syntax used in these examples is called the unabbreviated syntax. In many common cases, it is possible to write path expressions more concisely using an abbreviated syntax, as explained in .

The abbreviated syntax permits the following abbreviations:

Here are some examples of path expressions that use the abbreviated syntax:

One way to construct a sequence is by using the comma operator, which evaluates each of its operands and concatenates the resulting sequences, in order, into a single result sequence. Empty parentheses can be used to denote an empty sequence.

A sequence may contain duplicate atomic values or nodes, but a sequence is never an item in another sequence. When a new sequence is created by concatenating two or more input sequences, the new sequence contains all the items of the input sequences and its length is the sum of the lengths of the input sequences.

Here are some examples of expressions that construct sequences:

A range expression can be used to construct a sequence of consecutive integers. Each of the operands of the to operator is converted as though it was an argument of a function with the expected parameter type xs:integer?. If either operand is an empty sequence, or if the integer derived from the first operand is greater than the integer derived from the second operand, the result of the range expression is an empty sequence. If the two operands convert to the same integer, the result of the range expression is that integer. Otherwise, the result is a sequence containing the two integer operands and every integer between the two operands, in increasing order.

A filter expression consists simply of a primary expression followed by zero or more predicates. The result of the filter expression consists of the items returned by the primary expression, filtered by applying each predicate in turn, working from left to right. If no predicates are specified, the result is simply the result of the primary expression. The ordering of the items returned by a filter expression is the same as their order in the result of the primary expression. Context positions are assigned to items based on their ordinal position in the result sequence. The first context position is 1.

Here are some examples of filter expressions:

XQuery 1.1 provides the following operators for combining sequences of nodes:

All these operators eliminate duplicate nodes from their result sequences based on node identity. If ordering mode is ordered, the resulting sequence is returned in document order; otherwise it is returned in implementation-dependent order.

If an operand of union, intersect, or except contains an item that is not a node, a type error is raised .

Here are some examples of expressions that combine sequences. Assume the existence of three element nodes that we will refer to by symbolic names A, B, and C. Assume that the variables $seq1, $seq2 and $seq3 are bound to the following sequences of these nodes:

Then:

In addition to the sequence operators described here, includes functions for indexed access to items or sub-sequences of a sequence, for indexed insertion or removal of items in a sequence, and for removing duplicate items from a sequence.

The value comparison operators are eq, ne, lt, le, gt, and ge. Value comparisons are used for comparing single values.

The first step in evaluating a value comparison is to evaluate its operands. The order in which the operands are evaluated is implementation-dependent. Each operand is evaluated by applying the following steps, in order:

Next, if possible, the two operands are converted to their least common type by a combination of type promotion and subtype substitution. For example, if the operands are of type hatsize (derived from xs:integer) and shoesize (derived from xs:float), their least common type is xs:float.

Finally, if the types of the operands are a valid combination for the given operator, the operator is applied to the operands. The combinations of atomic types that are accepted by the various value comparison operators, and their respective result types, are listed in together with the operator functions that define the semantics of the operator for each type combination. The definitions of the operator functions are found in .

Informally, if both atomized operands consist of exactly one atomic value, then the result of the comparison is true if the value of the first operand is (equal, not equal, less than, less than or equal, greater than, greater than or equal) to the value of the second operand; otherwise the result of the comparison is false.

If the types of the operands, after evaluation, are not a valid combination for the given operator, according to the rules in , a type error is raised .

Here are some examples of value comparisons:

The general comparison operators are =, !=, <, <=, >, and >=. General comparisons are existentially quantified comparisons that may be applied to operand sequences of any length. The result of a general comparison that does not raise an error is always true or false.

A general comparison is evaluated by applying the following rules, in order:

When evaluating a general comparison in which either operand is a sequence of items, an implementation may return true as soon as it finds an item in the first operand and an item in the second operand that have the required magnitude relationship. Similarly, a general comparison may raise a dynamic error as soon as it encounters an error in evaluating either operand, or in comparing a pair of items from the two operands. As a result of these rules, the result of a general comparison is not deterministic in the presence of errors.

Here are some examples of general comparisons:

Node comparisons are used to compare two nodes, by their identity or by their document order. The result of a node comparison is defined by the following rules:

Here are some examples of node comparisons:

An element constructor creates an element node. A direct element constructor is a form of element constructor in which the name of the constructed element is a constant. Direct element constructors are based on standard XML notation. For example, the following expression is a direct element constructor that creates a book element containing an attribute and some nested elements:

If the element name in a direct element constructor has a namespace prefix, the namespace prefix is resolved to a namespace URI using the statically known namespaces. If the element name has no namespace prefix, it is implicitly qualified by the default element/type namespace. Note that both the statically known namespaces and the default element/type namespace may be affected by namespace declaration attributes found inside the element constructor. The namespace prefix of the element name is retained after expansion of the QName, as described in . The resulting expanded QName becomes the node-name property of the constructed element node.

In a direct element constructor, the name used in the end tag must exactly match the name used in the corresponding start tag, including its prefix or absence of a prefix.

In a direct element constructor, curly braces { } delimit enclosed expressions, distinguishing them from literal text. Enclosed expressions are evaluated and replaced by their value, as illustrated by the following example:

The above query might generate the following result (whitespace has been added for readability to this result and other result examples in this document):

Since XQuery uses curly braces to denote enclosed expressions, some convention is needed to denote a curly brace used as an ordinary character. For this purpose, a pair of identical curly brace characters within the content of an element or attribute are interpreted by XQuery as a single curly brace character (that is, the pair "{{" represents the character "{" and the pair "}}" represents the character "}".) Alternatively, the character references { and } can be used to denote curly brace characters. A single left curly brace ("{") is interpreted as the beginning delimiter for an enclosed expression. A single right curly brace ("}") without a matching left curly brace is treated as a static error .

The result of an element constructor is a new element node, with its own node identity. All the attribute and descendant nodes of the new element node are also new nodes with their own identities, even if they are copies of existing nodes.

XQuery allows an expression to generate a processing instruction node or a comment node. This can be accomplished by using a direct processing instruction constructor or a direct comment constructor. In each case, the syntax of the constructor expression is based on the syntax of a similar construct in XML.

A direct processing instruction constructor creates a processing instruction node whose target property is PITarget and whose content property is DirPIContents. The base-uri property of the node is empty. The parent property of the node is empty.

The PITarget of a processing instruction must not consist of the characters "XML" in any combination of upper and lower case. The DirPIContents of a processing instruction must not contain the string "?>".

The following example illustrates a direct processing instruction constructor:

A direct comment constructor creates a comment node whose content property is DirCommentContents. Its parent property is empty.

The DirCommentContents of a comment must not contain two consecutive hyphens or end with a hyphen. These rules are syntactically enforced by the grammar shown above.

The following example illustrates a direct comment constructor:

An alternative way to create nodes is by using a computed constructor. A computed constructor begins with a keyword that identifies the type of node to be created: element, attribute, document, text, processing-instruction, comment, or namespace.

For those kinds of nodes that have names (element, attribute, and processing instruction nodes), the keyword that specifies the node kind is followed by the name of the node to be created. This name may be specified either as a QName or as an expression enclosed in braces. When an expression is used to specify the name of a constructed node, that expression is called the name expression of the constructor.

The final part of a computed constructor is an expression enclosed in braces, called the content expression of the constructor, that generates the content of the node.

The following example illustrates the use of computed element and attribute constructors in a simple case where the names of the constructed nodes are constants. This example generates exactly the same result as the first example in :

An element node constructed by a direct or computed element constructor has an in-scope namespaces property that consists of a set of namespace bindings. The in-scope namespaces of an element node may affect the way the node is serialized (see ), and may also affect the behavior of certain functions that operate on nodes, such as fn:name. Note the difference between in-scope namespaces, which is a dynamic property of an element node, and statically known namespaces, which is a static property of an expression. Also note that one of the namespace bindings in the in-scope namespaces may have no prefix (denoting the default namespace for the given element). The in-scope namespaces of a constructed element node consist of the following namespace bindings:

In an element constructor, if two or more namespace bindings in the in-scope bindings would have the same prefix, then an error is raised if they have different URIs ; if they would have the same prefix and URI, duplicate bindings are ignored.

The following query serves as an example:

The in-scope namespaces of the resulting p:a element consists of the following namespace bindings:

The namespace bindings for p and q are added to the result element because their respective namespaces are used in the names of the element and its attributes. The namespace binding r="http://example.com/ns/r" is added to the in-scope namespaces of the constructed element because it is defined by a namespace declaration attribute, even though it is not used in a name.

No namespace binding corresponding to f="http://example.com/ns/f" is created, because the namespace prefix f appears only in the query prolog and is not used in an element or attribute name of the constructed node. This namespace binding does not appear in the query result, even though it is present in the statically known namespaces and is available for use during processing of the query.

Note that the following constructed element, if nested within a validate expression, cannot be validated:

The constructed element will have namespace bindings for the prefixes xsi (because it is used in a name) and xml (because it is defined for every constructed element node). During validation of the constructed element, the validator will be unable to interpret the namespace prefix xs because it is has no namespace binding. Validation of this constructed element could be made possible by providing a namespace declaration attribute, as in the following example:

The following clauses in FLWOR expressions bind values to variables: for, let, window, and count (in addition, a group by clause changes the values of variables that were previously bound.) In each case, binding of variables is governed by the following rules:

A for clause is used for iteration. Each variable in a for clause iterates over a sequence and is bound in turn to each item in the sequence.

If a for clause contains multiple variables, it is semantically equivalent to multiple for clauses, each containing one of the variables in the original for clause. In each of the semantically equivalent single-variable for clauses, the presence or absence of the outer keyword is the same as in the original for clause.

Examples:

In the remainder of this section, we define the semantics of a for clause containing a single variable and an associated expression (following the keyword in) whose value is called the binding sequence for that variable.

If a single-variable for clause is the initial clause in a FLWOR expression, it iterates over its binding sequence, binding the variable to each item in turn. The resulting sequence of variable bindings becomes the initial tuple stream that serves as input to the next clause of the FLWOR expression. If ordering mode is ordered, the order of tuples in the tuple stream preserves the order of the binding sequence; otherwise the order of the tuple stream is implementation-dependent.

If the binding sequence contains no items, the output tuple stream depends on the presence or absence of the outer keyword. If outer is specified, the output tuple stream consists of one tuple in which the variable is bound to an empty sequence. If outer is not specified, the output tuple stream consists of zero tuples.

The following examples illustrates tuple streams that are generated by initial for clauses:

A positional variable is a variable that is preceded by the keyword at. A positional variable may be associated with a variable that is bound in a for clause. In this case, as the main variable iterates over the items in its binding sequence, the positional variable iterates over the integers that represent the ordinal numbers of these items in the binding sequence, starting with one. Each tuple in the output tuple stream contains bindings for both the main variable and the positional variable. If the binding sequence is empty and outer is specified, the positional variable in the output tuple is bound to the integer zero. Positional variables always have the implied type xs:integer. The expanded QName of a positional variable must be distinct from the expanded QName of the main variable with which it is associated .

The following examples illustrate how a positional variable would have affected the results of the previous examples that generated tuples:

If a single-variable for clause is an intermediate clause in a FLWOR expression, its binding sequence is evaluated for each input tuple, given the bindings in that input tuple. Each input tuple generates zero or more tuples in the output tuple stream. Each of these output tuples consists of the original variable bindings of the input tuple plus a binding of the new variable to one of the items in its binding sequence.

For a given input tuple, if the binding sequence for the new variable in the for clause contains no items, the result depends on the presence or absence of the outer keyword. If outer is specified, the input tuple generates one output tuple, with the original variable bindings plus a binding of the new variable to an empty sequence. If outer is not specified, the input tuple generates zero output tuples (it is not represented in the output tuple stream.)

If the new variable introduced by a for clause has an associated positional variable, the output tuples generated by the for clause also contain bindings for the positional variable. In this case, as the new variable is bound to each item in its binding sequence, the positional variable is bound to the ordinal position of that item within the binding sequence, starting with one. Note that, since the positional variable represents a position within a binding sequence, the output tuples corresponding to each input tuple are independently numbered, starting with one. For a given input tuple, if the binding sequence is empty and outer is specified, the positional variable in the output tuple is bound to the integer zero.

If ordering mode is ordered, the tuples in the output tuple stream are ordered primarily by the order of the input tuples from which they are derived, and secondarily by the order of the binding sequence for the new variable; otherwise the order of the output tuple stream is implementation-dependent.

The following examples illustrates the effects of intermediate for clauses:

In the above examples, if ordering mode had been unordered, the output tuple streams would have consisted of the same tuples, with the same values for the positional variables, but the ordering of the tuples would have been implementation-dependent.

A for clause may contain one or more type declarations, identified by the keyword as. The semantics of type declarations are defined in .

The purpose of a let clause is to bind values to one or more variables. Each variable is bound to the result of evaluating an expression.

If a let clause contains multiple variables, it is semantically equivalent to multiple let clauses, each containing a single variable. For example, the clause

is semantically equivalent to the following sequence of clauses:

In the remainder of this section, we define the semantics of a let clause containing a single variable V and an associated expression E.

If a single-variable let clause is the initial clause in a FLWOR expression, it simply binds the variable V to the result of the expression E. The result of the let clause is a tuple stream consisting of one tuple with a single binding that binds V to the result of E. This tuple stream serves as input to the next clause in the FLWOR expression.

If a single-variable let clause is an intermediate clause in a FLWOR expression, it adds a new binding for variable V to each tuple in the input tuple stream. For each input tuple, the value bound to V is the result of evaluating expression E, given the bindings that are already present in that input tuple. The resulting tuples become the output tuple stream of the let clause.

The number of tuples in the output tuple stream of an intermediate let clause is the same as the number of tuples in the input tuple stream. The number of bindings in the output tuples is one more than the number of bindings in the input tuples, unless the input tuples already contain bindings for V; in this case, the new binding for V occludes (replaces) the earlier binding for V, and the number of bindings is unchanged.

A let clause may contain one or more type declarations, identified by the keyword as. The semantics of type declarations are defined in .

The following code fragment illustrates how a for clause and a let clause can be used together. The for clause produces an initial tuple stream containing a binding for variable $d to each department number found in a given input document. The let clause adds an additional binding to each tuple, binding variable $e to a sequence of employees whose department number matches the value of $d in that tuple.

Like a for clause, a window clause iterates over its binding sequence and generates a sequence of tuples. In the case of a window clause, each tuple represents a window. A window is a sequence of consecutive items drawn from the binding sequence. Each window is represented by at least one and at most nine bound variables. The variables have user-specified names, but their roles are as follows:

All variables in a window clause must have distinct names; otherwise a static error is raised .

The following is an example of a window clause that binds nine variables to the roles listed above. In this example, the variables are named $w, $s, $spos, $sprev, $snext, $e, $epos, $eprev, and $enext respectively. A window clause always binds the window variable, but typically binds only a subset of the other variables.

Windows are created by iterating over the items in the binding sequence, in order, identifying the start item and the end item of each window by evaluating the WindowStartCondition and the WindowEndCondition. Each of these conditions is satisfied if the effective boolean value of the expression following the when keyword is true. The start item of the window is an item that satisfies the WindowStartCondition (see and for a more complete explanation.) The end item of the window is the first item in the binding sequence, beginning with the start item, that satisfies the WindowEndCondition (again, see and for more details.) Each window contains its start item, its end item, and all items that occur between them in the binding sequence. If the end item is the start item, then the window contains only one item. If a start item is identified, but no following item in the binding sequence satisfies the WindowEndCondition, then the only keyword determines whether a window is generated: if only end is specified, then no window is generated; otherwise, the end item is set to the last item in the binding sequence and a window is generated.

In the above example, the WindowStartCondition and WindowEndCondition are both true(), which causes each tuple in the binding sequence to be in a separate window. Typically, the WindowStartCondition and WindowEndCondition are expressed in terms of bound variables. For example, the following WindowStartCondition might be used to start a new window for every item in the binding sequence that is larger than both the previous item and the following item:

The scoping rules for the variables bound by a window clause are as follows:

In a window clause, the keyword tumbling or sliding determines the way in which the starting item of each window is identified, as explained in the following sections.

A where clause serves as a filter for the tuples in its input tuple stream. The expression in the where clause, called the where-expression, is evaluated once for each of these tuples. If the effective boolean value of the where-expression is true, the tuple is retained in the output tuple stream; otherwise the tuple is discarded.

Examples:

The purpose of a count clause is to enhance the tuple stream with a new variable that is bound, in each tuple, to the ordinal position of that tuple in the tuple stream. The name of the new variable is specified in the count clause.

The output tuple stream of a count clause is the same as its input tuple stream, with each tuple enhanced by one additional variable that is bound to the ordinal position of that tuple in the tuple stream. However, if the name of the new variable is the same as the name of an existing variable in the input tuple stream, the new variable occludes (replaces) the existing variable of the same name, and the number of bound variables in each tuple is unchanged.

The following examples illustrate uses of the count clause:

A group by clause generates an output tuple stream in which each tuple represents a group of tuples from the input tuple stream. We will refer to the tuples in the input tuple stream as pre-grouping tuples, and the tuples in the output tuple stream as post-grouping tuples.

The post-grouping tuples have exactly the same variable-names as the pre-grouping tuples. The number of post-grouping tuples is less than or equal to the number of pre-grouping tuples. The group by clause assigns each pre-grouping tuple to a group, and generates one post-grouping tuple for each group. Subsequent clauses in the FLWOR expression see only the variable bindings in the post-grouping tuples; they no longer have access to the variable bindings in the pre-grouping tuples.

A group by clause consists of the keywords group by followed by one or more variables called grouping variables. The name of each grouping variable must be equal (by the eq operator on expanded QNames) to the name of a bound variable in the input tuple stream; otherwise a static error is raised . The process of group formation proceeds as follows:

The atomized value of a grouping variable is called a grouping key. For each pre-grouping tuple, the grouping keys are created by atomizing the values of the grouping variables. If the resulting value for any grouping variable consists of more than one item, a dynamic error is raised .

Each group of tuples produced by the above process results in one post-grouping tuple. The pre-grouping tuples from which the group is derived have equivalent grouping keys, but these keys are not necessarily identical (for example, the strings "Frog" and "frog" might be equivalent according to the collation in use.) In the post-grouping tuple, each grouping variable is bound to the value of one of the original grouping variables. The choice of which grouping variable is chosen is implementation-dependent.

In the post-grouping tuple generated for a given group, each non-grouping variable is bound to a sequence containing the concatenated values of that variable in all the pre-grouping tuples that were assigned to that group. If ordering mode is ordered, the values derived from individual tuples are concatenated in a way that preserves the order of the pre-grouping tuple stream; otherwise the ordering of these values is implementation-dependent.

The order in which tuples appear in the post-grouping tuple stream is implementation-dependent.

A group by clause rebinds all the variables in the input tuple stream. The scopes of these variables are not affected by the group by clause, but in post-grouping tuples the values of the variables represent group properties rather than properties of individual pre-grouping tuples.

Examples:

The purpose of an order by clause is to impose a value-based ordering on the tuples in the tuple stream. The output tuple stream of the order by clause contains the same tuples as its input tuple stream, but the tuples may be in a different order.

An order by clause contains one or more ordering specifications, called orderspecs, as shown in the grammar. For each tuple in the input tuple stream, the orderspecs are evaluated, using the variable bindings in that tuple. The relative order of two tuples is determined by comparing the values of their orderspecs, working from left to right until a pair of unequal values is encountered. If an orderspec specifies a collation, that collation is used in comparing values of type xs:string, xs:anyURI, or types derived from them (otherwise, the default collation is used in comparing values of these types). If an orderspec specifies a collation by a relative URI, that relative URI is resolved to an absolute URI using the base URI in the static context. If an orderspec specifies a collation that is not found in statically known collations, an error is raised .

The process of evaluating and comparing the orderspecs is based on the following rules:

For the purpose of determining their relative position in the ordering sequence, the greater-than relationship between two orderspec values W and V is defined as follows:

If T1 and T2 are two tuples in the input tuple stream, and V1 and V2 are the first pair of values encountered when evaluating their orderspecs from left to right for which one value is greater-than the other (as defined above), then:

If neither V1 nor V2 is greater-than the other for any pair of orderspecs for tuples T1 and T2, the following rules apply.

Examples:

The return clause is the final clause of a FLWOR expression. The return clause is evaluated once for each tuple in its input tuple stream, using the variable bindings in the respective tuples, in the order in which these tuples appear in the input tuple stream. The results of these evaluations are concatenated, as if by the comma operator, to form the result of the FLWOR expression.

The following example illustrates a FLWOR expression containing several clauses. The for clause iterates over all the departments in an input document named depts.xml, binding the variable $d to each department in turn. For each binding of $d, the let clause binds variable $e to all the employees in the given department, selected from another input document named emps.xml (the relationship between employees and departments is represented by matching their deptno values). Each tuple in the resulting tuple stream contains a pair of bindings for $d and $e ($d is bound to a department and $e is bound to a set of employees in that department). The where clause filters the tuple stream, retaining only those tuples that represent departments having at least ten employees. The order by clause orders the surviving tuples in descending order by the average salary of the employees in the department. The return clause constructs a new big-dept element for each surviving tuple, containing the department number, headcount, and average salary.