XQuery Basics

This is a method of running queries on XML. They are an extension of XPath and therefore every XPath is an XQuery expression.

For additional XQuery examples see this lecture video

Syntax

For the following tree:

graph
$doc -->|students|a[ ]
a -->|student|$s
$s -->|name| ''Anna''
$s -->|module| ''COMP207''

an XQuery could look like the following:

let $doc := doc("students.xml")
for $s in $doc/students/student
where $s/module = "COMP207"
return $s/name

This is case sensitive.

Clause	Requirement
`for` & `let`	Any number, interleaved. Some interpreters require at least one `let`.
`where`	Optional
`return`	Mandatory

Therefore we can get the following smallest expressions depending on the implementation:

return <greeting>Hello World!</greeting>

let $hello := <greeting>Hello World!</greeting>
return $hello

let
- for variable in XQuery expression
- Assigns the result of an expression to a variable.
for
- let variable := XQuery expression
- For each item in the result of the expression assign it the the variable.
where
- where condition 1, condition 2, ...
- If all the conditions are true then execute the return clause.
- You can also use the following syntax:
```
  where some/every $var in XQuery expression satisfies condition
```
Comparisons between expressions $s1/name = $s2/name may not have the tags removed.