The basic unit of evaluation in the XQuery language is the expression. A query contains expressions that can be made up of a number of sub-expressions, which may themselves be composed from other sub-expressions. This chapter explains the XQuery syntax, and covers the most basic types of expressions that can be used in queries: literals, variables, function calls, and comments.
A query can range in complexity from a single expression such as
2+3, to a complex composite expression like a FLWOR. Within a FLWOR, there may be other expressions, such as
$prodDept = "ACC", which is a comparison expression, and
doc("catalog.xml")/catalog/product, which is a path expression. Within these expressions are further expressions, such as
"ACC", which is a literal, and
$prodDept, which is a variable reference. Every expression evaluates to a sequence, which may be a single item (for example, an atomic value or node), the empty sequence, or multiple items.
The categories of expressions available are summarized in Table 3-1, along with a reference to the chapter or section that covers them.
|Category||Description||Operators or keywords||Chapter/Section|
|Primary||The basics: literals, variables, function calls, and parenthesized expressions||Chapter 3|
|Comparison||Comparison based on value, node identity, or document order||“Comparison Expressions”|
|String concatenation ...|