Lesson 8 — Querying XML

XPath Basics

XPath is a query language for navigating an XML document's tree structure and selecting specific elements or values, similar to how a file path navigates a folder structure.

Basic XPath Syntax

An XPath expression reads much like a file system path, using slashes to move between levels of the tree, and can select elements based on their attributes.

XPath — Selecting elements 
/library/book                  Select every <book> directly under <library>
/library/book/title             Select every <title> inside a <book>
//title                          Select every <title> anywhere in the document
/library/book[@id='2']          Select the <book> with an id attribute of "2"
/library/book[1]                Select the first <book> element

Common Mistakes

  • Confusing a single slash (/) — direct child — with a double slash (//) — any descendant at any depth.
  • Forgetting that XPath indices are 1-based (the first item is [1]), not 0-based like most programming languages.
  • Writing an attribute selector without the @ symbol, e.g. book[id='2'] instead of book[@id='2'].
  • Assuming XPath expressions are case-insensitive — they are case-sensitive, matching XML's own case sensitivity.

Professional Tip

Unlike most programming languages, XPath indices start at 1, not 0. This is a common source of off-by-one errors for developers coming from a typical programming background.

Your Turn

Given the library XML from the tree structure lesson, write XPath expressions to select all book titles, and to select only the second book element.

Mini Quiz

What does the // syntax mean in an XPath expression?