CS145 - Spring 2003
Introduction to Databases

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Homework 3 Questions and Answers

• When asking a question about OTC problem text or answer choices, please quote the relevant portions of the assignment.
• After you complete a submission, please keep a copy of the entire Web page with your score and submission certificate. In case of data loss, you may be asked to present the page for your highest-scoring submission in order to restore your high score.

• On the question asking for the pair of XPath and XQuery expressions that were not equivalent for a valid document, there were two correct answers, but only one was accepted. The question was corrected by 4pm Tue 29 Apr 2003. Sorry about the error in this question, and thank you to Kurt Miller, Donald Sze, and Manu Kumar for catching it.

  For students who were around after the 28 Apr 2003 problem session, here is a corrected explanation of multiple predicates in brackets:

  • As before, conditions are conjunctive; when multiple brackets appear, all must pass.
  • As before, the number in brackets is an abbreviation, so that [3] is equivalent to [position() = 3], an ordinary condition.
  • Correction: The brackets are not commutative; they are apparently applied in left-to-right order. That is, foo[a][b][c] is apparently interpreted as ((((foo)[a])[b])[c]).
  position() is not counted relative to foo, but to the results of the expression so far. For example, if position() occurs in condition b, it counts across the result of (foo[a]). As a result,
  • /Bookstore/Book[3] means "from the sequence of Book elements that are children of the document element Bookstore, return the third one." (Return the third Book in the Bookstore.)
  • /Bookstore/Book[3][@Price=70] means "from the sequence of Book elements that are children of the document element Bookstore, return the third one if additionally, it has an attribute Price with value 70." (Return the third Book in the Bookstore if its Price is 70.)
  • /Bookstore/Book[@Price=70][3] means "from the sequence of Book elements that are children of the document element Bookstore, and which additionally have an attribute Price with value 70, return the third one." (Return the third Book in the Bookstore that has a Price of 70.)
  Sorry to all students misled by the incorrect interpretation given earlier.

  (This interpretation is drawn loosely from the normative examples in the XPath 1.0 Recommendation and the XPath 2.0 draft, not the language grammar or specification text.)

1. Why are some constants in XQuery surrounded by apostrophes, some not?
   XQuery requires either apostrophes or quotation marks around literal strings (your choice, for each string), and forbids apostrophes or quotation marks for literal numbers (integers and floating point).
   (Reference: XQuery working draft 15 Nov 2002, Section 3.1.1 or A.1.1.)
2. Does an XQuery expression need to be surrounded by braces { } in a return clause?
   
   • If the expression is inside XML tags, yes; otherwise, the expression is treated as a literal string. The braces "escape" you back from XML output into XQuery processing. For example,
     for $i in 1 return <one> $i </one> returns <one> $i </one>, and
     for $i in 1 return <one> { $i } </one> returns <one> 1 </one>.
   • If the expression is the entire return clause, no. For example, for $i in 1 return input()//Title evaluates the XPath expression, returning all elements named Title from the XQuery expression's input.
   (Reference: XQuery working draft 15 Nov 2002, Sections A.2, 3.7.1.)
3. What does it mean for an XPath expression to have a contiguous sequence of [condition a][condition b] = Any point where a condition in brackets is permitted, multiple conditions, each in brackets, are permitted one after another. Where multiple conditions appear, all of them apply.
4. What do parent:: and self:: mean in XPath (and XQuery)?
   Relative to a node, parent:: means to consider the node one step toward the root of the tree, rather than the default of considering all children nodes. self:: means to consider the same node, rather than the default of considering all the children nodes. So, for example, where Element or child::Element means to select any child elements named Element, parent::Element means to select any element named Element one step "up" (toward the root of the tree), and self::Element means to select any node itself, if it is named Element. The designation before the double colon is called the axis, as mentioned in class.
5. What does node() test in XPath (and XQuery)? The function node() selects (matches) any node. For example, where self::Element selects a node itself only if it is named Element, self::node() selects itself as long as it is a node.

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