Filtering Results of Queries

Sometimes you want to retrieve only those elements that meet a certain condition. For example, you might want information about a particular book. In this case, you can include a filter in your query. You enclose filters in brackets ( [ ] ).

The following figure shows how the XPath processor interprets a query with a filter:

The following topics provide details about filters:

Quotation Marks in Filters

Suppose you define the following filter:

[title="History of Trenton"]

More Filter Examples

Following is another example of a query with a filter clause. This query returns book elements if the price of the book is greater than 25 dollars:

/bookstore/book[price > 25]

You can apply constraints and branching to a query by specifying a filter clause. The filter contains a query, which is called the subquery. The subquery evaluates to a Boolean value, or to a numeric value. The XPath processor tests each element in the current context to see if it satisfies the subquery. The result includes only those elements that test true for the subquery.

The XPath processor always evaluates filters with respect to a context. For example, the expression book[author] means for every book element that is found in the current context, determine whether the book element contains an author element. For example, the following query returns all books in the current context that contain at least one excerpt:

book[excerpt] 

You can specify any number of filters in any level of a query expression. Empty filters
( [ ] ) are not allowed.

A query that contains one or more filters returns the rightmost element that is not in a filter clause. For example:

book[excerpt]/author[degree] 

The following query finds each book child of the current context that has an author with at least one degree:

book[author/degree] 

Following is a query that finds all child elements of the current context with specialty attributes:

*[@specialty]