Path, Predicates, and XQuery

So far, the examples of predicates have been simple path expressions, comparison expressions, and numbers. In fact, any expression is allowed in a predicate, making it a very flexible construct. For example, predicates can contain function calls, as in:

  doc("catalog.xml")/catalog/product[contains(@dept, "A")]

which returns all product children whose dept attribute contains the letter A. They can contain conditional expressions, as in:

  doc("catalog.xml")/catalog/product[if ($descFilter)
                     then desc else true()]

which filters product elements based on their desc child only if the variable $descFilter is true. They can also contain expressions that combine sequences, as in:

  doc("catalog.xml")/catalog/product[* except number]

which returns all product children that have at least one child other than number. General comparisons with multiple values can be used, as in:

  doc("catalog.xml")/catalog/product[@dept = ("ACC", "WMN", "MEN")]

which returns products whose dept attribute value is any of those three values. This is similar to a SQL "in" clause.

To retrieve every third product child of catalog, you could use the expression:

  doc("catalog.xml")/catalog/product[position() mod 3 = 0]

because it selects all the products whose position is divisible by 3.

Predicates can even contain path expressions that themselves have predicates. For example:

  doc("catalog.xml")/catalog/product[*[3][self::colorChoices]]

can be used to find all product elements whose third child element is colorChoices. The *[3][self::colorChoices] is part of a separate path expression that is itself within a predicate. *[3] selects the third child element of product, and [self:: colorChoices] is a way of testing the name of the current context element.

Predicates are not limited to use with path expressions. They can be used with any sequence. For example:

  (1 to 100)[. mod 5 = 0]

can be used to return the integers from 1 to 100 that are divisible by 5. Another example is:

  (@price, 0.0)[1]

which selects the price attribute if it exists, or the decimal value 0.0 otherwise.