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XQuery:

An XML query

language

by D. Chamberlin

The World Wide Web Consortium has

convened a working group to design a query

language for Extensible Markup Language

(XML) data sources. This new query language,

called XQuery, is still evolving and has been

described in a series of drafts published by

the working group. XQuery is a functional

language comprised of several kinds of

expressions that can be nested and

composed with full generality. It is based on

the type system of XML Schema and is

designed to be compatible with other XML-

related standards. This paper explains the

need for an XML query language, provides a

tutorial overview of XQuery, and includes

several examples of its use.

Increasingly, Extensible Markup Language (XML)

is considered the format of choice for the exchange

of information among various applications on the

Internet. The popularity of XML is due in large part

to its ﬂexibility for representing many kinds of in-

formation. The use of tags makes XML data self-de-

scribing, and the extensible nature of XML makes it

possible to deﬁne new kinds of documents for spe-

cialized purposes. As the importance of XML has in-

creased, a series of standards has grown up around

it, many of which were deﬁned by the World Wide

Web Consortium (W3C).

For example, XML Sche-

provides a notation for deﬁning new types of

elements and documents; XML Path Language

(XPath)

provides a notation for selecting elements

within an XML document; and Extensible Stylesheet

Language Transformations (XSLT)

provides a no-

tation for transforming XML documents from one

representation to another.

XML makes it possible for applications to exchange

data in a standard format that is independent of stor-

age. For example, one application may use a native

XML storage format, whereas another may store data

in a relational database. Since XML is emerging as

a standard for data exchange, it is natural that que-

ries among applications should be expressed as que-

ries against data in XML format. This use gives rise

to a requirement for a query language designed ex-

pressly for XML data sources. In October 1999, W3C

convened the XML Query Working Group

for the

purpose of designing such a query language, to be

called XQuery.

XML data are different from relational data in sev-

eral important respects that inﬂuence the design of

a query language. Relational data tend to have a reg-

ular structure, which allows the descriptive meta-data

for these data to be stored in a separate catalog. XML

data, in contrast, are often quite heterogeneous, and

distribute their meta-data throughout the document.

XML documents often contain many levels of nested

elements, whereas relational data are “ﬂat.” XML

documents have an intrinsic order, whereas relational

data are unordered except where an ordering can

be derived from data values. Relational data are usu-

ally “dense” (nearly every column has a value), and

tion. Copying in printed form for private use is permitted with-

out payment of royalty provided that (1) each reproduction is done

without alteration and (2) the Journal reference and IBM copy-

right notice are included on the ﬁrst page. The title and abstract,

but no other portions, of this paper may be copied or distributed

royalty free without further permission by computer-based and

other information-service systems. Permission to republish any

other portion of this paper must be obtained from the Editor.

597

relational systems often represent missing informa-

tion by a special null value.

XML data, in contrast,

are often “sparse” and can represent missing infor-

mation simply by the absence of an element. For

these and other reasons, existing relational query lan-

guages are not directly suitable for querying XML

data.

The design of XQuery is still in progress. The XML

Query Working Group has published working drafts

of several documents that describe the current state

of the design. Of these, perhaps the most important

is XQuery 1.0: An XML Query Language,

which con-

tains a syntax and informal description of the lan-

guage. The working group has also published a list

of requirements,

a description of the data model

that underlies the language,

a formal semantic de-

scription,

a list of functions and operators,

and

a collection of use cases that illustrate applications

of the language.

Each of these documents is up-

dated from time to time as the design of XQuery

evolves. This paper is based on the most recent

XQuery design at the time of its publication, but since

this design is still changing, the documents refer-

enced in this paragraph should be consulted for the

latest developments.

The design of XQuery has been subject to a number

of inﬂuences. Perhaps the most important of these

is compatibility with existing W3C standards, includ-

ing Schema, XSLT, XPath, and XML itself. XPath, in

particular, is so important and so closely related that

XQuery is deﬁned as a superset of XPath. The over-

all design of XQuery is based on a language proposal

called Quilt.

Quilt, in turn, was inﬂuenced by the

functional approach of Object Query Language

(OQL),

by the keyword-based syntax of Structured

Query Language (SQL),

and by previous XML query

language proposals including XQL,

XML-QL,

and

Lorel.

It is an objective of the XML Query Working Group

to deﬁne two syntaxes for XQuery: one that is ex-

pressed in XML, and one that is optimized for hu-

man writing and understanding. This paper describes

only the human-oriented version of XQuery.

The initial design of XQuery is focused only on in-

formation retrieval and does not provide facilities

for updating existing XML documents. The XML

Query Working Group may consider the addition

of an update facility after completing the design of

the ﬁrst version of XQuery.

This paper describes the data model on which

XQuery is based, and then presents an overview of

the XQuery language in the form of a series of ex-

amples. This paper is not intended to provide a rig-

orous or exhaustive deﬁnition of the language. The

reader is referred to Reference 7 for an XQuery syn-

tax and a more complete language description.

Data model

Formally, the input and output of XQuery are de-

ﬁned in terms of a data model, described in Refer-

ence 9. The query data model provides an abstract

representation of one or more XML documents or

document fragments. The data model is based on

the notion of a sequence. A sequence is an ordered

collection of zero or more items. An item may be a

node or an atomic value. An atomic value is an in-

stance of one of the built-in data types deﬁned by XML

Schema, such as strings, integers, decimals, and dates.

A node conforms to one of seven node kinds, which

include element, attribute, text, document, comment,

processing instruction, and namespace nodes. A

node may have other nodes as children, thus form-

ing one or more node hierarchies. Some kinds of

nodes, such as element and attribute nodes, have

names or typed values, or both. A typed value is a

sequence of zero or more atomic values. Nodes have

identity (that is, two nodes may be distinguishable

even though their names and values are the same),

but atomic values do not have identity. Among all

the nodes in a hierarchy there is a total ordering

called document order, in which each node appears

before its children. Document order corresponds to

the order in which the nodes would appear if the

node hierarchy were represented in XML format.

Document order between nodes in different hierar-

chies is implementation-deﬁned but must be consis-

tent; that is, all the nodes in one hierarchy must be

ordered either before or after all the nodes in an-

other hierarchy.

Sequences may be heterogeneous; that is, they may

contain mixtures of various types of nodes and atomic

The design of

XQuery has been

subject to a number

of inﬂuences.

CHAMBERLIN IBM SYSTEMS JOURNAL, VOL 41, NO 4, 2002

598

values. However, a sequence never appears as an

item in another sequence. All operations that cre-

ate sequences are deﬁned to “ﬂatten” their operands

so that the result of the operation is a single-level

sequence. There is no distinction between an item

and a sequence of length one—in other words, a

node or atomic value is considered to be identical

to a sequence of length one containing that node or

atomic value.

Sequences of length zero are valid and are some-

times used to represent missing or unknown infor-

mation, in much the same way that null values are

used in relational systems.

In addition to sequences, the query data model de-

ﬁnes a special value called the error value, which is

the result of evaluating an expression that contains

an error. An error value may not be combined in a

sequence with any other value.

Input XML documents can be transformed into the

query data model by a process called schema vali-

dation, which parses the document, validates it

against a particular schema, and represents it as a

hierarchy of nodes and atomic values, labeled with

type information derived from the schema. If an in-

put document does not have a schema, it is validated

against a permissive default schema that assigns ge-

neric types—nodes are labeled anyType and atomic

values are labeled anySimpleType. The process of

schema validation is described in more detail in Ref-

erence 3.

The result of a query may be transformed from the

query data model into an XML representation by a

process called serialization. The details of serializa-

tion are beyond the scope of this paper. It is worth

noting that the result of a query is not always a well-

formed XML document. For example, a query might

return an atomic value such as the number 47, or a

sequence of elements with no common parent.

Example data

To illustrate the query data model and provide a ba-

sis for later examples, we consider a small XML da-

tabase that contains data from an on-line auction,

itemno seller description reserve-price end-date

(ALL ITEM ELEMENTS

HAVE SIMILAR STRUCTURE)

Figure 1 Data model representation of items.xml

items.xml

items

item

status

item

status

TTT T T

E EE E

IBM SYSTEMS JOURNAL, VOL 41, NO 4, 2002 CHAMBERLIN

599

based loosely on Use Case R in Reference 12. The

database consists of two XML documents named

items.xml and bids.xml.

The items.xml document contains a root element

named items, which in turn contains an item element

for each item currently for sale at the auction. Each

item element has a status attribute and subelements

named itemno, seller, description, reserve-price,

and end-date. The reserve-price element names a

minimum selling price set by the owner, and the

end-date element indicates the ending date of the

auction.

The bids.xml document contains a root element

named bids, which in turn contains a bid element

for each bid that has been placed for an item. Each

bid element has subelements named itemno, bidder,

bid-amount, and bid-date.

Figures 1 and 2 show the data model representations

of the items.xml and bids.xml documents, respec-

tively (including only a representative item and a rep-

resentative bid). In the ﬁgures, the circles labeled

D, E, A, and T represent document, element, at-

tribute, and text nodes, respectively.

Expressions

We now describe expressions in XQuery.

Basics. Like XML and XPath, XQuery is a case-sen-

sitive language, and all its keywords are made up of

lowercase characters. Detailed rules for lexing and

parsing XQuery are described in Reference 7. Char-

acters enclosed between “{--” and “--}” are con-

sidered to be comments and are ignored during query

processing (except, of course, inside a quoted string,

where they are considered to be part of the string).

XQuery is a functional language, which means that

it is made up of expressions that return values and

do not have side effects. XQuery has several kinds

of expressions, most of which are composed from

lower-level expressions, combined by operators or

keywords. XQuery expressions are fully composable,

that is, where an expression is expected, any kind of

expression may be used. As noted earlier, the value

itemno bidder bid-amount bid-date

(ALL BID ELEMENTS

HAVE SIMILAR STRUCTURE)

Figure 2 Data model representation of bids.xml

bids.xml

bids

bid

TTT T

EE E

CHAMBERLIN IBM SYSTEMS JOURNAL, VOL 41, NO 4, 2002

600

of an expression, in general, is a heterogeneous se-

quence of nodes and atomic values.

The simplest kind of XQuery expression is a literal,

which represents an atomic value. The following are

several examples of literals:

47 is a literal of type integer

4.7

is a literal of type decimal because it con-

tains a decimal point

4.7E3 is a literal of type double because it con-

tains an exponent

''47'' is a literal of type string (single quotes

are allowed inside double-quoted strings)

'47 ' is a literal of type string (double quotes

are allowed inside single-quoted strings)

Atomic values of other types may be created by call-

ing constructors. A constructor is a function that cre-

ates a value of a particular type from a string con-

taining a lexical representation of the desired type.

In general, a constructor has the same name as the

type it constructs. The following example uses a con-

structor to create a value of type

date:

date(''2002-5-31'')

Any XQuery expression may be enclosed in paren-

theses. Parentheses are useful for making explicit the

order in which an expression should be evaluated.

The following examples of arithmetic expressions

show how parentheses can be used to control the pre-

cedence of operators. Arithmetic expressions are dis-

cussed in more detail in a later subsection.

(2 ⫹ 4) ⴱ 5 has the value 30 because the sub-

expression

(2 ⫹ 4) is evaluated

ﬁrst

2 ⫹ 4 ⴱ 5 has the value 22 because ⴱ has a

higher precedence than

⫹

The comma operator concatenates two values to

form a sequence. Sequences are often enclosed in

parentheses as explicit delimiters, although this is not

required. An empty pair of parentheses denotes an

empty sequence. Since sequences cannot be nested,

the comma operator constructs a sequence consist-

ing of all the items in its left operand, followed by

all the items in its right operand. A sequence can

also be constructed by the

to operator, which returns

a sequence consisting of all the integers between its

left operand and its right operand, inclusive. The fol-

lowing examples illustrate construction of sequences:

1, 2, 3 is a sequence of three values

(1, 2, 3) is identical to 1, 2, 3

((1, 2), ⵹,3) is identical to 1, 2, 3

1to3 is identical to 1, 2, 3

A variable in XQuery is a name that begins with a

dollar sign. A variable may be bound to a value and

used in an expression to represent that value. One

way to bind a variable is by means of a LET expres-

sion, which binds one or more variables and then

evaluates an inner expression. The value of the LET

expression is the result of evaluating the inner expres-

sion with the variables bound. The following exam-

ple illustrates a LET expression that returns the se-

quence 1, 2, 3:

let $start :⫽ 1, $stop :⫽ 3

return $start to $stop

A LET expression is a special case of a FLWR (for,

let, where, return) expression, which provides addi-

tional ways to bind variables. FLWR expressions are

described in more detail later.

Another simple form of XQuery expression is a func-

tion call. XQuery provides a core function library,

described in Reference 11, and a mechanism

whereby users can deﬁne additional functions, de-

scribed in the next section. Function calls in XQuery

employ the usual notation in which the arguments

of the function are enclosed in parentheses. The fol-

lowing example calls the core library function

substring to extract the ﬁrst six characters from a

string:

substring(''Martha Washington'' ,1,6)

Path expressions. Path expressions in XQuery are

based on the syntax of XPath.

A path expression

consists of a series of steps, separated by the slash

character (“/”). The result of each step is a sequence

of nodes. The value of the path expression is the node

sequence that results from the last step in the path.

Each step is evaluated in the context of a particular

node, called the context node. In general, a step can

be any expression that returns a sequence of nodes.

One important kind of step, called an axis step, can

be thought of as beginning at the context node and

moving through the node hierarchy in a particular

direction, called an axis. As the axis step moves along

the designated axis, it selects nodes that satisfy a se-

lection criterion. The selection criterion can select

nodes based on their names, their positions with re-

IBM SYSTEMS JOURNAL, VOL 41, NO 4, 2002 CHAMBERLIN

601

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