How to Use for Loop as a Predicate in XQuery and XPath to Filter Unique Years

Learn how to efficiently filter unique year instances in XQuery and XPath using the `for loop` as a predicate. --- This video is based ...

In XQuery and XPath, using a for loop as a predicate can be a bit tricky, but you can achieve filtering unique years using a combination of loops and the distinct-values function. Here’s how you can do it step-by-step:

Example XML Data

Assuming you have the following XML structure containing years:

<records>
    <record>
        <year>2020</year>
        <event>Event A</event>
    </record>
    <record>
        <year>2021</year>
        <event>Event B</event>
    </record>
    <record>
        <year>2020</year>
        <event>Event C</event>
    </record>
    <record>
        <year>2022</year>
        <event>Event D</event>
    </record>
</records>

Step 1: Extract Unique Years

You can use distinct-values to get unique years from the XML data. Here’s how to do it in XQuery:

let $records := 
    <records>
        <record>
            <year>2020</year>
            <event>Event A</event>
        </record>
        <record>
            <year>2021</year>
            <event>Event B</event>
        </record>
        <record>
            <year>2020</year>
            <event>Event C</event>
        </record>
        <record>
            <year>2022</year>
            <event>Event D</event>
        </record>
    </records>

let $uniqueYears := distinct-values($records/record/year)

return
    <unique-year-list>
        {
            for $year in $uniqueYears
            return <year>{$year}</year>
        }
    </unique-year-list>

Explanation

1. Define XML Data: The XML structure is stored in the $records variable.
2. Retrieve Unique Years: The distinct-values function extracts unique years from the records.
3. Loop Through Unique Years: The for loop iterates over each unique year, creating a new <year> element for each one.

Output

The output will be:

<unique-year-list>
    <year>2020</year>
    <year>2021</year>
    <year>2022</year>
</unique-year-list>

Using XPath

If you want to use XPath to filter unique years, you can use a similar approach with XPath 2.0 or higher:

distinct-values(/records/record/year)

This XPath expression will return a sequence of unique years directly.

Conclusion

By combining the distinct-values function with a for loop, you can effectively filter and display unique years from your XML data in XQuery. This method leverages the strengths of both XQuery and XPath for efficient data manipulation.