XML query

introduction An XML document can represent almost anything, and users of an XML query language expect it to perform useful queries on whatever they have stored in XML. 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.

XQuery is defined in terms of the XQuery 1.0 and XPath 2.0 Data Model [XQ-DM], which represents the parsed structure of an XML document as an ordered, labeled tree in which nodes have identity and may be associated with simple or complex types. XQuery is a functional language

XML vs. Relational Data { row: { name: “John”, phone: 3634 }, row: { name: “Sue”, phone: 6343 }, row: { name: “Dick”, phone: 6363 } } row name phone “John”3634 “Sue” “Dick” Relation … in XML

Relational to XML Data A relation instance is basically a tree with: – Unbounded fanout at level 1 (i.e., any # of rows) – Fixed fanout at level 2 (i.e., fixed # fields) XML data is essentially an arbitrary tree – Unbounded fanout at all nodes/levels – Any number of levels – Variable # of children at different nodes, variable path lengths

Query Language for XML Must be high-level; “SQL for XML” Must conform to XSchema – But also work in absence of schema info Support simple and complex/nested datatypes Support universal and existential quantifiers, aggregation Operations on sequences and hierarchies of doc structures Capability to transform and create XML structures

XQuery Influenced by XML-QL, Lorel, Quilt, YATL – Also, XPath and XML Schema Reads a sequence of XML fragments or atomic values and returns a sequence of XML fragments or atomic values – Inputs/outputs are objects defined by XML-Query data model, rather than strings in XML syntax

Overview of XQuery Path expressions Element constructors FLWOR (“flower”) expressions – Several other kinds of expressions as well, including conditional expressions, list expressions, quantified expressions, etc. Expressions evaluated w.r.t. a context: – Context item (current node) – Context position (in sequence being processed) – Context size (of the sequence being processed) – Context also includes namespaces, variables, functions, date, etc.

Path Expressions Examples: Bib/paper Bib/book/publisher Bib/paper/author/lastname Given an XML document, the value of a path expression p is a set of objects

Path Expression Examples Doc = &o1 &o12&o24&o29 &o43 &o70&o71 &96 &243 &206 &25 “Serge” “Abiteboul” 1997 “Victor” “Vianu” paper book paper references author title year http author title publisher author title page firstnamelastname firstname lastname firstlast Bib &o44&o45&o46 &o47&o48 &o49 &o50 &o51 &o52 Bib/paper = Bib/book/publisher = Bib/paper/author/lastname = Bib/paper = Bib/book/publisher = Bib/paper/author/lastname = Note that order of elements matters!

Element Construction An XQuery expression can construct new values or structures Example: Consider the path expressions from the previous slide. – Each of them returns a newly constructed sequence of elements – Key point is that we don’t just return existing structures or atomic values; we can re-arrange them as we wish into new structures

Data Model In the XQuery data model, every document is represented as a tree of nodes. The kinds of nodes that may occur are: document, element, attribute, text, name- space, processing instruction, and comment. An item is a single node or atomic value. A series of items is known as a sequence. In XQuery, every value is a sequence

Literals and comments (: Hello World :)

Doc() function Returns entire document doc("books.xml")

Locating nodes A path expression consists of a series of one or more steps, separated by a slash, /, or double slash, //. doc("books.xml")/bib/book doc("books.xml")//book

Predicates Predicates are Boolean conditions that select a subset of the nodes computed by a step expression. XQuery uses square brackets around predicates. For instance, the following query returns only authors for which last="Stevens" is true: doc("books.xml")/bib/book/author[last="Stev ens"]

If a predicate contains a single numeric value, it is treated like a subscript. For instance, the following expression returns the first author of each book: (doc("books.xml")/bib/book/author)[1]

Creating Nodes document { Harold and the Purple Crayon Johnson Crockett HarperCollins Juvenile Books }

{ doc("books.xml")//title }

FLWOR Expressions FOR-LET-WHERE-ORDERBY-RETURN = FLWOR FOR / LET Clauses WHERE Clause ORDERBY/RETURN Clause List of tuples Instance of XQuery data model

FOR vs. LET FOR $x IN list-expr – Binds $x in turn to each value in the list expr LET $x = list-expr – Binds $x to the entire list expr – Useful for common sub-expressions and for aggregations

FOR vs. LET: Example FOR $x IN document("bib.xml") /bib/book RETURN $x FOR $x IN document("bib.xml") /bib/book RETURN $x Returns:... LET $x IN document("bib.xml") /bib/book RETURN $x LET $x IN document("bib.xml") /bib/book RETURN $x Returns:... Notice that result has several elements Notice that result has exactly one element

XQuery Example 1 Find all book titles published after 1995: FOR $x IN document("bib.xml") /bib/book WHERE $x/year > 1995 RETURN $x/title FOR $x IN document("bib.xml") /bib/book WHERE $x/year > 1995 RETURN $x/title Result: abc def ghi

XQuery Example 2 For each author of a book by Morgan Kaufmann, list all books she published: FOR $a IN distinct( document("bib.xml") /bib/book[publisher=“Morgan Kaufmann”]/author) RETURN $a, FOR $t IN /bib/book[author=$a]/title RETURN $t FOR $a IN distinct( document("bib.xml") /bib/book[publisher=“Morgan Kaufmann”]/author) RETURN $a, FOR $t IN /bib/book[author=$a]/title RETURN $t distinct = a function that eliminates duplicates (after converting inputs to atomic values)

Results for Example 2 Jones abc def Smith ghi Observe how nested structure of result elements is determined by the nested structure of the query.

XQuery Example 3 count = (aggregate) function that returns the number of elements FOR $p IN distinct(document("bib.xml")//publisher) LET $b := document("bib.xml")/book[publisher = $p] WHERE count($b) > 100 RETURN $p FOR $p IN distinct(document("bib.xml")//publisher) LET $b := document("bib.xml")/book[publisher = $p] WHERE count($b) > 100 RETURN $p For each publisher p - Let the list of books published by p be b Count the # books in b, and return p if b > 100

XQuery Example 4 Find books whose price is larger than average: LET $a=avg( document("bib.xml") /bib/book/price) FOR $b in document("bib.xml") /bib/book WHERE $b/price > $a RETURN $b LET $a=avg( document("bib.xml") /bib/book/price) FOR $b in document("bib.xml") /bib/book WHERE $b/price > $a RETURN $b

FLWOER Expressions for $b in doc("books.xml")//book where = "2000" return $b/title