Querying XML: XPath, XQuery, and XSLT Zachary G. Ives University of Pennsylvania CIS 550 – Database & Information Systems October 27, 2005 Some slide content courtesy of Susan Davidson, Dan Suciu, & Raghu Ramakrishnan

2 Reminders  Homework 4 due 11/3  XQuery  Project plan due 11/3  Milestones  Division of responsibilities  For non-RSS projects: proposal including scope, milestones, and what you plan to demonstrate  Recitation: Friday 11:30-12:30, Levine 512

3 Talks of Interest Today:  Anastassia Ailamaki, CMU, “FATES: Automatically-tuned Database Storage Management”, IRCS (3401 Walnut) Room 3PM Tomorrow as part of “DB & Information Retrieval Day”:  Anastassia Ailamaki, CMU, “StagedDB: Designing Database Servers for New Hardware Trends”, Wu and 11AM  Sam Madden, MIT, “Data Management for Next Generation Wireless Sensor Networks”, Wu and 12:30PM  Andrei Broder, Yahoo!, “The next stage in Web IR: From query based Information Retrieval to context driven Information Supply ”, Wu and 2:30PM

4 Querying XML How do you query a directed graph? a tree? The standard approach used by many XML, semistructured-data, and object query languages:  Define some sort of a template describing traversals from the root of the directed graph  In XML, the basis of this template is called an XPath

5 XPaths In its simplest form, an XPath is like a path in a file system: /mypath/subpath/*/morepath  The XPath returns a node set representing the XML nodes (and their subtrees) at the end of the path  XPaths can have node tests at the end, returning only particular node types, e.g., text(), processing-instruction(), comment(), element(), attribute()  XPath is fundamentally an ordered language: it can query in order-aware fashion, and it returns nodes in order

6 Sample XML Kurt P. Brown PRPL: A Database Workload Specification Language 1992 Univ. of Wisconsin-Madison Paul R. McJones The 1995 SQL Reunion Digital System Research Center Report SRC db/labs/dec/SRC html

7 XML Data Model Visualized Root ?xml dblp mastersthesis article mdate key authortitleyearschool editortitleyearjournalvolumeee mdate key 2002… ms/Brown92 Kurt P…. PRPL… 1992 Univ…. 2002… tr/dec/… Paul R. The… Digital… SRC… 1997 db/labs/dec attribute root p-i element text

8 Some Example XPath Queries  /dblp/mastersthesis/title  /dblp/*/editor  //title  //title/text()

9 Context Nodes and Relative Paths XPath has a notion of a context node: it’s analogous to a current directory  “.” represents this context node  “..” represents the parent node  We can express relative paths: subpath/sub-subpath/../.. gets us back to the context node  By default, the document root is the context node

10 Predicates – Selection Operations A predicate allows us to filter the node set based on selection-like conditions over sub-XPaths: /dblp/article[title = “Paper1”] which is equivalent to: /dblp/article[./title/text() = “Paper1”]

11 Axes: More Complex Traversals Thus far, we’ve seen XPath expressions that go down the tree (and up one step)  But we might want to go up, left, right, etc.  These are expressed with so-called axes:  self::path-step  child::path-stepparent::path-step  descendant::path-stepancestor::path-step  descendant-or-self::path-stepancestor-or-self::path-step  preceding-sibling::path-stepfollowing-sibling::path-step  preceding::path-stepfollowing::path-step  The previous XPaths we saw were in “abbreviated form”

12 Querying Order  We saw in the previous slide that we could query for preceding or following siblings or nodes  We can also query a node for its position according to some index:  fn:first(), fn:last()return index of 0 th & last element matching the last step:  fn:position()gives the relative count of the current node child::article[fn:position() = fn:last()]

13 Users of XPath  XML Schema uses simple XPaths in defining keys and uniqueness constraints  XQuery  XSLT  XLink and XPointer, hyperlinks for XML

14 XQuery A strongly-typed, Turing-complete XML manipulation language  Attempts to do static typechecking against XML Schema  Based on an object model derived from Schema Unlike SQL, fully compositional, highly orthogonal:  Inputs & outputs collections (sequences or bags) of XML nodes  Anywhere a particular type of object may be used, may use the results of a query of the same type  Designed mostly by DB and functional language people Attempts to satisfy the needs of data management and document management  The database-style core is mostly complete (even has support for NULLs in XML!!)  The document keyword querying features are still in the works – shows in the order-preserving default model

15 XQuery’s Basic Form  Has an analogous form to SQL’s SELECT..FROM..WHERE..GROUP BY..ORDER BY  The model: bind nodes (or node sets) to variables; operate over each legal combination of bindings; produce a set of nodes  “FLWOR” statement [note case sensitivity!]: for {iterators that bind variables} let {collections} where {conditions} order by {order-conditions}(older version was “SORTBY”) return {output constructor}

16 “Iterations” in XQuery A series of (possibly nested) FOR statements assigning the results of XPaths to variables for $root in document(“ for $sub in $root/rootElement, $sub2 in $sub/subElement, …  Something like a template that pattern-matches, produces a “binding tuple”  For each of these, we evaluate the WHERE and possibly output the RETURN template  document() or doc() function specifies an input file as a URI  Old version was “document”; now “doc” but it depends on your XQuery implementation

17 Two XQuery Examples { for $p in document(“dblp.xml”)/dblp/proceedings, $yr in $p/yr where $yr = “1999” return {$p} } for $i in document(“dblp.xml”)/dblp/inproceedings[author/text() = “John Smith”] return { $i/title/text() } { } { $i/crossref }

18 Nesting in XQuery Nesting XML trees is perhaps the most common operation In XQuery, it’s easy – put a subquery in the return clause where you want things to repeat! for $u in document(“dblp.xml”)/universities where $u/country = “USA” return { $u/title } { for $mt in $u/../mastersthesis where $mt/year/text() = “1999” and ____________ return $mt/title }

19 Collections & Aggregation in XQuery In XQuery, many operations return collections  XPaths, sub-XQueries, functions over these, …  The let clause assigns the results to a variable Aggregation simply applies a function over a collection, where the function returns a value (very elegant!) let $allpapers := document(“dblp.xml”)/dblp/article return { fn:count(fn:distinct-values($allpapers/authors)) } {for $paper in doc(“dblp.xml”)/dblp/article let $pauth := $paper/author return {$paper/title} { fn:count($pauth) } }

20 Collections, Ctd. Unlike in SQL, we can compose aggregations and create new collections from old: { let $avgItemsSold := fn:avg( for $order in document(“my.xml”)/orders/order let $totalSold = fn:sum($order/item/quantity) return $totalSold) return $avgItemsSold }

21 Distinct-ness In XQuery, DISTINCT-ness happens as a function over a collection  But since we have nodes, we can do duplicate removal according to value or node  Can do fn:distinct-values(collection) to remove duplicate values, or fn:distinct-nodes(collection) to remove duplicate nodes for $years in fn:distinct-values(doc(“dblp.xml”)//year/text() return $years

22 Sorting in XQuery  SQL actually allows you to sort its output, with a special ORDER BY clause (which we haven’t discussed, but which specifies a sort key list)  XQuery borrows this idea  In XQuery, what we order is the sequence of “result tuples” output by the return clause: for $x in document(“dblp.xml”)/proceedings order by $x/title/text() return $x

23 What If Order Doesn’t Matter? By default:  SQL is unordered  XQuery is ordered everywhere!  But unordered queries are much faster to answer XQuery has a way of telling the query engine to avoid preserving order:  unordered { for $x in (mypath) … }

24 Querying & Defining Metadata – Can’t Do This in SQL Can get a node’s name by querying node-name(): for $x in document(“dblp.xml”)/dblp/* return node-name($x) Can construct elements and attributes using computed names: for $x in document(“dblp.xml”)/dblp/*, $year in $x/year, $title in $x/title/text(), element node-name($x) { attribute {“year-” + $year} { $title } }

25 XQuery Summary Very flexible and powerful language for XML  Clean and orthogonal: can always replace a collection with an expression that creates collections  DB and document-oriented (we hope)  The core is relatively clean and easy to understand Turing Complete – we’ll talk more about XQuery functions soon

26 XSL(T): The Bridge Back to HTML  XSL (XML Stylesheet Language) is actually divided into two parts:  XSL:FO: formatting for XML  XSLT: a special transformation language  We’ll leave XSL:FO for you to read off if you’re interestedwww.w3.org  XSLT is actually able to convert from XML  HTML, which is how many people do their formatting today  Products like Apache Cocoon generally translate XML  HTML on the server side

27 A Different Style of Language  XSLT is based on a series of templates that match different parts of an XML document  There’s a policy for what rule or template is applied if more than one matches (it’s not what you’d think!)  XSLT templates can invoke other templates  XSLT templates can be nonterminating (beware!)  XSLT templates are based on XPath “match”es, and we can also apply other templates (potentially to “select”ed XPaths)  Within each template, we describe what should be output  (Matches to text default to outputting it)

28 An XSLT Stylesheet This is DBLP …

29 Results of XSLT Stylesheet Paper1 Smith Chakrabarti Gray Paper2 This Is DBLP Paper1 Smith Paper2 Chakrabarti Gray

30 What XSLT Can and Can’t Do  XSLT is great at converting XML to other formats  XML  diagrams in SVG; HTML; LaTeX  …  XSLT doesn’t do joins (well), it only works on one XML file at a time, and it’s limited in certain respects  It’s not a query language, really  … But it’s a very good formatting language  Most web browsers (post Netscape 4.7x) support XSLT and XSL formatting objects  But most real implementations use XSLT with something like Apache Cocoon  You may want to use XSL/XSLT for your projects – see for the spec

31 Querying XML We’ve seen three XML manipulation formalisms today:  XPath: the basic language for “projecting and selecting” (evaluating path expressions and predicates) over XML  XQuery: a statically typed, Turing-complete XML processing language  XSLT: a template-based language for transforming XML documents  Each is extremely useful for certain applications!

32 Views in SQL and XQuery  A view is a named query  We use the name of the view to invoke the query (treating it as if it were the relation it returns) SQL: CREATE VIEW V(A,B,C) AS SELECT A,B,C FROM R WHERE R.A = “123” XQuery: declare function V() as element(content)* { for $r in doc(“R”)/root/tree, $a in $r/a, $b in $r/b, $c in $r/c where $a = “123” return {$a, $b, $c} } SELECT * FROM V, R WHERE V.B = 5 AND V.C = R.C for $v in V()/content, $r in doc(“r”)/root/tree where $v/b = $r/b return $v Using the views:

33 What’s Useful about Views Providing security/access control  We can assign users permissions on different views  Can select or project so we only reveal what we want! Can be used as relations in other queries  Allows the user to query things that make more sense Describe transformations from one schema (the base relations) to another (the output of the view)  The basis of converting from XML to relations or vice versa  This will be incredibly useful in data integration, discussed soon… Allow us to define recursive queries

34 Materialized vs. Virtual Views  A virtual view is a named query that is actually re-computed every time – it is merged with the referencing query CREATE VIEW V(A,B,C) AS SELECT A,B,C FROM R WHERE R.A = “123”  A materialized view is one that is computed once and its results are stored as a table  Think of this as a cached answer  These are incredibly useful!  Techniques exist for using materialized views to answer other queries  Materialized views are the basis of relating tables in different schemas SELECT * FROM V, R WHERE V.B = 5 AND V.C = R.C

35 Views Should Stay Fresh  Views (sometimes called intensional relations) behave, from the perspective of a query language, exactly like base relations (extensional relations)  But there’s an association that should be maintained:  If tuples change in the base relation, they should change in the view (whether it’s materialized or not)  If tuples change in the view, that should reflect in the base relation(s)

36 View Maintenance and the View Update Problem  There exist algorithms to incrementally recompute a materialized view when the base relations change  We can try to propagate view changes to the base relations  However, there are lots of views that aren’t easily updatable:  We can ensure views are updatable by enforcing certain constraints (e.g., no aggregation), but this limits the kinds of views we can have! AB BC R S ABC R⋈SR⋈S delete?

37 Next Time  Can we have views in XML over tables in relations?  … Or vice versa?  What other things can we use views for