1 XQuery Values FLWR Expressions Other Expressions

2 XQuery uXQuery extends XPath to a query language that has power similar to SQL. uXQuery is an expression language. wLike relational algebra --- any XQuery expression can be an argument of any other XQuery expression. wUnlike RA, with the relation as the sole datatype, XQuery has a subtle type system.

3 The XQuery Type System 1.Atomic values : strings, integers, etc. uAlso, certain constructed values like true(), date(“ ”). 2.Nodes. uSeven kinds. uWe’ll only worry about four, on next slide.

4 Some Node Types 1.Element Nodes are like nodes of semistructured data. uDescribed by !ELEMENT declarations in DTD’s. 2.Attribute Nodes are attributes, described by !ATTLIST declarations in DTD’s. 3.Text Nodes = #PCDATA. 4.Document Nodes represent files.

5 Example Document … <BEER name = “Bud” soldBy = “JoesBar SuesBar … ”/> …

6 Example Nodes BARS PRICE BEERBAR name = “JoesBar” theBeer = “Miller” theBeer = “Bud” SoldBy = “…” name = “Bud” Green = element Gold = attribute Purple = text

7 Document Nodes uForm: document(“ ”). uEstablishes a document to which a query applies. uExample: document(“/usr/ullman/bars.xml”)

8 XQuery Values uItem = node or atomic value. uValue = ordered sequence of zero or more items. uExamples: 1.() = empty sequence. 2.(“Hello”, “World”) 3.(“Hello”, 2.50, 10)

9 Nesting of Sequences Ignored uA value can, in principle, be an item of another value. uBut nested list structures are expanded. uExample: ((1,2),(),(3,(4,5))) = (1,2,3,4,5) = 1,2,3,4,5. uImportant when values are computed by concatenating other values.

10 FLWR Expressions 1.One or more for and/or let clauses. 2.Then an optional where clause. 3.A return clause.

11 Semantics of FLWR Expressions uEach for creates a loop. wlet produces only a local definition. uAt each iteration of the nested loops, if any, evaluate the where clause. uIf the where clause returns TRUE, invoke the return clause, and append its value to the output.

12 FOR Clauses for in,... uVariables begin with $. uA for-variable takes on each item in the sequence denoted by the expression, in turn. uWhatever follows this for is executed once for each value of the variable.

13 Example: FOR for $beer in return {$beer} u$beer ranges over the name attributes of all beers in our example document. uResult is a list of tagged names, like Bud Miller... “Expand the en- closed string by replacing variables and path exps. by their values.”

14 LET Clauses let :=,... uValue of the variable becomes the sequence of items defined by the expression. uNote let does not cause iteration; for does.

15 Example: LET let $d := document(“bars.xml”) let $beers := return {$beers} uReturns one element with all the names of the beers, like: Bud Miller …

16 Following IDREF’s uXQuery (but not XPath) allows us to use paths that follow attributes that are IDREF’s. uIf x denotes a sequence of one or more IDREF’s, then x =>y denotes all the elements with tag y whose ID’s are one of these IDREF’s.

17 Example uFind all the beer elements where the beer is sold by Joe’s Bar for less than uStrategy: 1.$beer will for-loop over all beer elements. 2.For each $beer, let $joe be either the Joe’s- Bar element, if Joe sells the beer, or the empty sequence if not. 3.Test whether $joe sells the beer for < 3.00.

18 Example: The Query let $d := document(”bars.xml”) for $beer in $d/BARS/BEER let $joe := let $joePrice := where $joePrice < 3.00 return {$beer} Attribute soldBy is of type IDREFS. Follow each ref to a BAR and check if its name is Joe’s Bar. Find that PRICE subelement of the Joe’s Bar element that represents whatever beer is currently $beer. Only pass the values of $beer, $joe, $joePrice to the RETURN clause if the string inside the PRICE element $joePrice is < 3.00

19 Order-By Clauses uFLWR is really FLWOR: an order-by clause can precede the return. uForm: order by wWith optional ascending or descending. uThe expression is evaluated for each output element. uDetermines placement in output sequence.

20 Example: Order-By uList all prices for Bud, lowest first. let $d := document(“bars.xml”) for $p in order by $p return { $p }

21 Predicates uNormally, conditions imply existential quantification. uExample: means “all the bars that have a name.” uExample: = means “Joe and Sue have at least one price in common.”

22 Other Operators uUse Fortran comparison operators to compare atomic values only. weq, ne, gt, ge, lt, le. uArithmetic operators: +, -, *, div, idiv, mod. wApply to any expressions that yield arithmetic or date/time values.

23 Effective Boolean Values uThe effective boolean value (EBV) of an expression is: 1.The actual value if the expression is of type boolean. 2.FALSE if the expression evaluates to 0, “” [the empty string], or () [the empty sequence]. 3.TRUE otherwise.

24 EBV Examples has EBV TRUE or FALSE, depending on whether the name attribute is ”JoesBar”. has EBV TRUE if some bar is named the Golden Rail, and FALSE if there is no such bar.

25 Boolean Operators uE 1 and E 2, E 1 or E 2, not(E ), if (E 1 ) then E 2 else E 3 apply to any expressions. uTake EBV’s of the expressions first. uExample: not(3 eq 5 or 0) has value TRUE. uAlso: true() and false() are functions that return values TRUE and FALSE.

26 Quantifier Expressions some $x in E 1 satisfies E 2 1.Evaluate the sequence E 1. 2.Let $x (any variable) be each item in the sequence, and evaluate E 2. 3.Return TRUE if E 2 has EBV TRUE for at least one $x. uAnalogously: every $x in E 1 satisfies E 2

27 Document Order uComparison by document order: >. uExample: << is true iff the Bud element appears before the Miller element in the document $d.

28 Set Operators uunion, intersect, except operate on sequences of nodes. wMeanings analogous to SQL. wResult eliminates duplicates. wResult appears in document order.