XQL OQL and SQL Xia Tang Sixin Qian Shijun Shen Feb 18 2000 XQL XML Query Language XQL is a notation for addressing and filtering the elements and text of XML documents W3C XQL XML Query Language XQL is designed specifically for XML documents XQL is a proposed extension to the XSL pattern syntax XQL is concise simple and powerful What is an XML Query Questions XQL What is a database The database is a set of one or more XML documents What is the input to a query A query is given a list of input nodes from one or more documents What is the result of a The result of a query is a list of query XML document nodes which may serve as the basis for further queries Approaches Using Document Object Model subtrees as the basis for a query Using data dictionaries or repository directory structures to identify nodes to be queried Using a URL identify documents XQL notations Selects the current node Selects the parent of current node Selects the document node a expr Select only those nodes matching a which also Satisfy the expression expr a n Selects the nth matching node matching a a b For each node matching a add the nodes matching b to the result a b For each node matching a add the descendant nodes matching b to the result b Returns elements in the entire document matching b a b All nodes matching a or b XQL notation A simple string is interpreted as an element name Example table this query specification returns all table elements XQL notation The child operator indicates hierarchy Front author This query returns author elements that are children of front elements XQL notation The equals operator The content of an element or the value of an attribute may be specified using Front author Bob return all authors with the name Bob that are children of the front element XQL notation Attribute names are preceded by the They are treated as children of the elements to which they belong Front author address type email Find the type attribute with email for all address elements XQL notation The descendant operator indicates any number of intervening levels Bookstore title Find all title elements one or more levels deep in the bookstore arbitrary descendants XQL notation The filter operator filters the set of nodes to its left based on the conditions inside the brackets Front author address type email returns addresses each of these addresses must have a attribute called type with the value email XQL notation Brackets are also used for subscripts which indicate position within a document Conditions and subscripts both uses of brackets may occur in the same query Section level 3 1 to 2 It returns the first three level 3 sections Example xml version 1 0 invoicecollection invoice customer Bob CA customer entries n 2 entries invoice invoice customer John CA customer entries n 2 entries invoice invoicecollection Query customer Result xql result customer Bob CA customer customer John CA customer xql result XQL Expressions Terms node attr node text comment pi Selects all child elements with node name is node Selects all child elements Selects the attribute attr Selects all attributes Matches an element node Matches a text node Matches a comment Matches a processing instruction node XQL Expressions Namespaces and names In attribute names the attribute prefix comes before the namespace prefix my book Find all book elements with the prefix my XQL Expression Comparisons A set of binary comparison operators is available for comparing numbers and strings and returning Boolean results and are allowed short cuts for lt le gt and ge XQL Expressions Boolean and Set Operators Boolean expressions can be used within subqueries Multiple conditions may be combined using Boolean operators and set operators XQL Expressions Grouping operator Parentheses can be used to group collection operators for clarity or where the normal precedence is inadequate to express an operation XQL Expressions Functions collection functions Taken from XSL pattern language text The text contained within the element This concatenates the text of all of the element s descendants It does not include the tag names or any attribute values comment values etc It trims the string as discussed in text Returns a string value Returns a type cast version of the value of an element See Datatypes If data typing is not supported or a data type is not provided returns the same as text nodeType Returns a number to indicate the type of the node element 1 attribute 2 text 3 PI 7 comment 8 document 9 nodeName The tag name of the node including the namespace prefix Returns a string XQL Expression Collection Function search context of the Invocation instance evaluates to a subset of the search context Pure Function Ignore the search context evaluates to either a constant value or to a value that depends only on the function s parameters XQL Expressions Extensible Functions To allow user defined functions to be written XQL provides a function called function Joins To allow subtrees from one data source to be inserted into another document subtree subject to the join conditions XQL Expressions Renaming Operator The nodes in a list may be renamed using the renaming operator may also be used to adjust namespaces in query results Precedence of Operators Production Grouping Filter Subscript Bang Path Comparison Intersection Union Negation Conjunction Disjunction Operator s eq ne lt le gt ge ieq ine ilt ile igt ige intersect union not and or XQL s important key features XML documents can be queried like a database XQL is based on of XML so it allows for powerful queries to be simply expressed It uses a string syntax that allows queries to be used in URL s or embedded in attributes Links http www w3 org TandS QL QL98 pp xql html http www w3 org Style XSL Group 1998 09 X QL proposal html