DB2 10.5 for Linux, UNIX, and Windows
Expressions
Expressions are the basic building blocks of a query. Expressions can be used alone or in combination with other expressions to form complex queries. DB2® XQuery supports several kinds of expressions for working with XML data.
- Expression evaluation and processing
A number of operations are often included in the processing of expressions. These operations include extracting atomic values from nodes, using type promotion and subtype substitution to obtain values of an expected type, and computing the Boolean value of a sequence. - Primary expressions
Primary expressions are the basic primitives of the language. They include literals, variable references, parenthesized expressions, context item expressions, constructors, and function calls. - Path expressions
Path expressions identify nodes within an XML tree. Path expressions in DB2 XQuery are based on the syntax of XPath 2.0. - Predicates
A predicate filters a sequence by retaining the qualifying items. A predicate consists of an expression, called a predicate expression, that is enclosed in square brackets ([]). - Sequence expressions
Sequence expressions construct, filter, and combine sequences of items. Sequences are never nested. For example, combining the values 1, (2, 3), and ( ) into a single sequence results in the sequence (1, 2, 3). - Arithmetic expressions
Arithmetic expressions perform operations that involve addition, subtraction, multiplication, division, and modulus. - Comparison expressions
Comparison expressions compare two values. XQuery provides three kinds of comparison expressions: value comparisons, general comparisons, and node comparisons. - Logical expressions
Logical expressions use the operators and and or to compute a Boolean value (true or false). - Constructors
Constructors create XML structures within a query. XQuery provides constructors for creating element nodes, attribute nodes, document nodes, text nodes, processing instruction nodes, and comment nodes. XQuery provides two kinds of constructors: direct constructors and computed constructors. - FLWOR expressions
FLWOR expressions iterate over sequences and bind variables to intermediate results. FLWOR expressions are useful for computing joins between two or more documents, restructuring data, and sorting the result. - Conditional expressions
Conditional expressions use the keywords if, then, and else to evaluate one of two expressions based on whether the value of a test expression is true or false. - Quantified expressions
Quantified expressions return true if some or every item in one or more sequences satisfies a specific condition. The value of a quantified expression is always true or false. - Cast expressions
A cast expression creates a new value of a specific type based on an existing value. - Castable expressions
Castable expressions test whether a value can be cast to a specific data type. If the value can be cast to the data type, the castable expression returns true. Otherwise, the expression returns false. - Transform expression and updating expressions
To update existing XML data with DB2 XQuery, use updating expressions within the modify clause of a transform expression.