XML & Databases. Tutorial. 3. XPath Queries. Universität Konstanz. Database & Information Systems Group Prof. Marc H. Scholl

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1 XML & Databases Tutorial Christian Grün, Database & Information Systems Group University of, Winter 2007/08

2 XPath Introduction navigational access to XML documents sub-language in XQuery, XSLT, or XPointer current standard: XPath 2.0 Example queries * 3 numeric calculations concat( any, string ) string operations /a/b/c/d/e location paths //a[something!= nothing ] location paths with predicates -a[b[@c][/d e][1]] getting cryptic Seite 2

3 XPath Context Sets Location Steps a location steps processes sets of XML nodes (so-called context sets) a location step expects a context set as input it returns a result set, which may be evaluated by another location step context sets may contain redundant XML contents if they are serialized (see example) in XPath implementations, context sets are often represented as simple ID lists B A E C D F G <A> <B><C/><D/> </B> <E> <F/><G/> </E> </A> descendant::node() B A E C D F G <B><C/><D/></B>, <C/>, <D/>, <E><F/><G/></E>, <F/>, <G/>, Seite 3

4 XPath Location Steps Syntax the axis defines the navigational direction of a location step: attribute, descendant, child, following, parent, the node test restricts the results to specific node types. the most important ones are: node() accepts all nodes text() accepts all text nodes * accepts all elements (tags) name accepts nodes with a certain tag name the predicates, represented in squared brackets, serve as filters. they may contain any valid XPath expression [child::node()] accepts a node if it has children [n] returns the n th node Seite 4

5 XPath Axes preceding preceding-sibling ancestor-or-self ancestor parent following following-sibling self attribute child descendant Seite 5

6 XPath Axes Some Notes most important axes are child and descendant many publications concentrate on these two axes the following and preceding axes are rarely used the child axis doesn t return attributes as these are treated like element properties some context set and axis combinations will never yield results e.g.: attribute or text nodes and child steps usually, axes are combined with node tests to limit the number of result nodes: /descendant-or-self::node() returns all nodes /descendant-or-self::yo only returns nodes with the tag name yo Seite 6

7 XPath Node Tests & Predicates Filtering Nodes result sets can be refined with node tests and predicates Examples descendant::text() returns all descendant text nodes attribute::id returns all id attributes child::*[text()] returns all elements which have text nodes as children Positional Predicates the position of results can be specified via numeric values and the position() and last() function: child::xyz[1] returns the first xyz tag ancestor::node()[last()] returns the root node Seite 7

8 XPath Functions Extending XPath Features similar to other programming languages, XPath includes some pre-defined functions to allow operations which can t be expressed via location paths Examples count(//whatever) returns the number of found whatever tags string-length(any/location/path) returns the length of the atomized context set (atomization: see lecture) contains(any/location/path, howdy ) returns true if the substring howdy is found in the atomized context set see for more information Seite 8

9 Java Coding Strings Strings in Java are objects, not native types still: Java Strings are efficient: same strings are internally stored only once hash code as e.g. used in Collections is cached avoid explicit string construction: new String("I'm a string") "I'm a string use equals() method to compare strings: "!!!" == "!!! yields true but String s = "!"; s + "!!" == "!!!" yields false Seite 9

10 Java Coding Strings vs. StringBuilder the StringBuilder provides an internal buffer for constructing strings similar to StringBuffer, but no synchronization avoid this: String tmp = ""; for(int i = 0; i < 1000; i++) tmp += "x"; rather try this: StringBuilder sb = new StringBuilder(); for(int i = 0; i < 1000; i++) sb.append("x"); return sb.tostring(); Seite 10

11 Java Coding Call-by-Reference take care if you pass on objects: int[] array = { 1, 2 }; correctadd(array, 3); System.out.println(array[0]); wrongadd(array, 3); System.out.println(array[0]); void correctadd(int[] array, int val) { array[0] += val; } void wrongadd(int[] array, int val) { array = new int[array.length]; for(int i = 0; i < array.length; a++) array[i] += val; } Seite 11

12 Java Coding Elegance database context: performance has high priority avoid unnecessary code boolean values: if(x == true) return true; else return false; return x; ternary expressions: if(str == null) return ""; else return str; return str == null? "" : str; Seite 12

13 Java Coding Array Handling Java offers the method System.arraycopy( ) to copy arrays Example int[] input = new int[5000]; for(int i = 0; i < 5000; i++) input[i] = i; int[] output = new int[5000]; for(int i = 0; i < 5000; i++) output[i] = input[i]; System.arraycopy(input, 0, output, 0, input.length); Seite 13

14 Java Coding Collections Java Collections offer a convenient toolbox implementations often surpass equivalent C structures but: check which data structure you use: Hash!= Tree!= Linked List! = Map no support for native values (int, double, ) have a look into the sources and implement your own structure! Seite 14