XQL (XML Query Language) Jonathan Robie (Software AG) Eduard Derksen (CSCIO) Peter Fankhauser (GMD-IPSI) Ed Howland (DEGA) Gerald Huck (GMD-IPSI) Ingo Macherius (GMD-IPSI) Makoto Murata (Fuji Xerox) Michael Resnick (Object Design, Incorporated) Harald Schöning (Software AG)

XQL vs. SQL What is the database? –SQL: A set of tables –XQL: A set of one or more XML documents. What is the query language? –SQL: A query language that uses the structure of tables as a basic model. –XQL: A query language that uses the structure of XML documents as a basic model. What is the input to a query? –SQL: The FROM clause determines the tables which are examined by the query. –XQL: A query is given a list of input nodes from one or more documents. What is the result of a query? –SQL: The result of a query is a table containing a set of rows; this table may serve as the basis for further queries. –XQL: The result of a query is a list of XML document nodes, which may serve as the basis for further queries.

XQL Expressions Terms Namespaces and Names Comparisons Hierarchy and Filters Boolean and Set Operators Grouping Operator Sequence

XQL Expressions (cond.) Functions Extensible Functions References (not incorporated into XQL) Joins Renaming Operator Precedence of Operators

Learn XQL by Examples Terms: –A simple string is interpreted as an element name: Query example: author. –Attribute names begin with Query example: Hierarchy: –Child operator “/” Query example: /novel/author –Paths are described from the top down and usually the right-most element on the path is returned –Descendant operator ("//") indicates any number of intervening levels Example: novel//address –Shows addresses anywhere within.

Learn XQL by Examples (cond.) Filters: –Filter operator ("[ ]") filters the set of nodes to its left based on the conditions inside the brackets. Example: –Returns addresses: each address must have an attribute called "type" with the value " ”. Boolean Operations and Comparisons: –Boolean operators can be used to combine conditions. Example: –novel/author =’Tom’ Set Operations: –Can be used to combine Terms or other XQL expressions. Example: q1 union q2 (also can be written as q1 | q2) Example: q1 intersect q2

Learn XQL by Examples (cond.) Namespaces and Names: –Names may be associated with namespace prefixes –A namespace prefix can be declared using a variable. –Example: b := " //b:book The first line declares "b" to be a variable equivalent to the namespace URI " The second line searches for all elements in this namespace. Grouping Operator: –Useful to group results. –Example: //book {.//author} The element to the left of the {} is used to group the results of the query within the braces.

Learn XQL by Examples (cond.) Sequence Operators: –a before b (returns a list of all "a"s that precede a "b”) –a after b (returns a list of all "a"s that occur after a "b”) –a, b(returns a list containing all "a"s, followed by all "b"s) XQL operators generally maintain document order. Useful for specifying order in return lists. Functions: –XQL provide some functions. –Example: element(), element(‘name’) –returns elements in the context. If a name argument is supplied, returns the elements with the given name. count()

Learn XQL by Examples (cond.) Extensible Functions: –Allow users to write their own functions –Functions are passed the list of nodes in the current context. If the function has parameters, these are passed as strings. –The result of a function call is also a nodelist. Example: –User can add a function that computes the average for a list of values and call it in an XQL query –average(property//price) – Renaming Operator –Nodes in a list can be renamed using renaming operator "->". Example: //book->a:book Rename the book into a new namespace a

Learn XQL by Examples (cond.) References: –XSL provides "id()" function, which returns the element containing a given id. Example: –Return the node pointed to by an HREF attribute in an element Actually it is a join. But this syntax less complex than join. –XQL extends to incorporate any kind of link, not just ID/IDREF. Example: Function ref() returns the node (nodes) to which an XPointer or HTML HREF points Can specify parameters to this function –Example: "table") –Return the referenced node only if it is a 'table" element; otherwise, will return null.

Learn XQL by Examples (cond.) Joins: –Combine information from multiple sources to create one unified view. –Example: /book { isbn | title | author | //review { reviewer | comments } } –Only correct when the database consists only of this one book and this one review. /book[$i:=isbn] { isbn | title | author | //review[isbn=$i] { reviewer | comments }} –using correlation variables to restrict the reviews to those that have the same ISBN number as the book.