From http://www.w3schools.com (Copyright Refsnes Data)

XQuery Syntax

XQuery is case-sensitive and XQuery elements, attributes, and variables must be valid XML names.

XQuery Basic Syntax Rules

Some basic syntax rules:

• XQuery is case-sensitive
• XQuery elements, attributes, and variables must be valid XML names
• An XQuery string value can be in single or double quotes
• An XQuery variable is defined with a $ followed by a name, e.g. $bookstore
• XQuery comments are delimited by (: and :), e.g. (: XQuery Comment :)

XQuery Conditional Expressions

"If-Then-Else" expressions are allowed in XQuery.

Look at the following example:

for $x in doc("books.xml")/bookstore/book
return	if ($x/@category="CHILDREN")
	then <child>{data($x/title)}</child>
	else <adult>{data($x/title)}</adult>

Notes on the "if-then-else" syntax: parentheses around the if expression are required. else is required, but it can be just else ().

The result of the example above will be:

<adult>Everyday Italian</adult>
<child>Harry Potter</child>
<adult>Learning XML</adult>
<adult>XQuery Kick Start</adult>

XQuery Comparisons

In XQuery there are two ways of comparing values.

1. General comparisons: =, !=, <, <=, >, >=

2. Value comparisons: eq, ne, lt, le, gt, ge

The difference between the two comparison methods are shown below.

Look at the following XQuery expressions:

$bookstore//book/@q > 10

The expression above returns true if any q attributes
have values greater than 10.

$bookstore//book/@q gt 10

The expression above returns true if there is only one
q attribute returned by the expression, and its value
is greater than 10. If more than one q is returned,
an error occurs.

From http://www.w3schools.com (Copyright Refsnes Data)