Native XML databases - Making Sense of NoSQL - page 107

FLWOR with loop
without any “let”
statements

Figure 5.6 A sample
FLWOR statement in XQuery
that shows the for, where,
order, and return
statements. Many
structures in XQuery, such
as the where and order
by statements, are similar
to SQL, so many SQL
developers quickly learn
basic XQuery statements.

Get all “sale” nodes in this XML file

“Where” clause similar
to SQL returns only
amounts over 15
“Order by” clause to
change sort order

for $sale in doc('sales.xml')//sale

where $sale/amount >15
order by $sale/amount descending
return
$sale

“Return” returns
a sequence of
sale nodes

experts in query language design. Some of these experts assisted in developing stan-
dards for SQL , and in time found SQL inappropriate for querying nontabular data.
The W3C query standards group was charged with creating a single query language
that would work well for all use cases, which included business as well as text docu-
ments, books, and articles. Beginning with a list of relational and document query lan-
guages and a set of 70 use cases for structured and unstructured data sets, the W3C
group embarked on an multiyear mission to define the XQuery language.
XQuery was designed to be a flexible language that allowed each of the 70 use
cases to be queried and run on multiple processors, and to be easy to learn, parse, and
debug. To accomplish its task, it borrowed concepts from many other query systems.
This rigorous process resulted in a query language that's widely adopted and used in
many products. These XQuery products include not only native XML databases, but
also tools for in-memory data transformation and integration tools.

XQ UERY — A FUNCTIONAL PROGRAMMING LANGUAGE
XQuery is defined as a functional programming language because its focus is the parallel
transformation of sequences of data items using functions. Note that we'll cover the
topic of functional programming in depth in chapter 11. With XQuery, functions can
be passed as parameters to other functions. XQuery has many features not found in
SQL that are used for the efficient transformation of hierarchical XML data. For exam-
ple, XQuery allows you to call recursive functions and returns not only tables but any
other tree-like data structures. XQuery can return simple XML or a sequence of items
including JSON or graph structures. Due to its functional nature, XQuery can be eas-
ier to execute on multiple CPU systems.
The parallel processing power of XQuery is the FLWOR statement . FLWOR stands for
for, let, where, order, and return , as shown in figure 5.6. Unlike the for loops found in pro-
cedural languages such as Java or .Net, FLWOR statements can execute in independent
parallel processes and run on a large number of parallel CPU s or different processors.

XQ UERY — CONSISTENT WITH WEB STANDARDS
XQuery is designed to be consistent with other W3C standards beyond XPath. For
example, XQuery shares data types with other XML standards such as XML Schema,
XProc, and Schematron. Because of this standardization, XQuery implementations
tend to be more portable than applications that are ported between SQL databases.