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, there are further expressions, such as "ACC", which is a literal, and $prodDept, which is a variable reference.
The categories of expressions available are summarized in Table 3-1, along with the number of the chapter that covers them. Every expression evaluates to a sequence, which may be a single atomic value, a single node, the empty sequence, or multiple atomic values and/or nodes.
Table 3-1. Categories of expressions
|
Category |
Description |
Operators or keywords |
Chapter |
|---|---|---|---|
|
Primary |
The basics: literals, variables, function calls, and parenthesized expressions |
3 | |
|
Comparison |
Comparison based on value, node identity, or document order |
|
3 |
|
Conditional |
If-then-else expressions |
|
3 |
|
Logical |
Boolean |
|
3 |
|
Path |
Selecting nodes from XML documents |
|
4 |
|
Constructor |
Adding XML to the results |
|
5 |
|
FLWOR |
Controlling the selection and processing of nodes |
|
6 |
|
Quantified |
Determining whether sequences fulfill specific conditions |
|
6 |
|
Sequence-related |
Creating and combining sequences |
|
9 |
|
Type-related |
Casting and validating values based on type |
|
11, 14 |
|
Arithmetic |
Adding, subtracting, multiplying, and dividing |
|
16 |