XQuery

Joins

One of the major benefits of FLWORs is that they can easily join data from multiple sources. For example, suppose you want to join information from your product catalog (catalog.xml) and your order (order.xml). You want a list of all the items in the order, along with their number, name, and quantity. Example 6-13 shows a FLWOR that performs this join.

Example 6-13. Two-way join in a predicate

Query
for $item in doc("order.xml")//item,
    $product in doc("catalog.xml")//product[number = $item/@num]
return <item num="{$item/@num}"
             name="{$product/name}"
             quan="{$item/@quantity}"/>
Results
<item num="557" name="Fleece Pullover" quan="1"/>
<item num="563" name="Floppy Sun Hat" quan="1"/>
<item num="443" name="Deluxe Travel Bag" quan="2"/>
<item num="784" name="Cotton Dress Shirt" quan="1"/>
<item num="784" name="Cotton Dress Shirt" quan="1"/>
<item num="557" name="Fleece Pullover" quan="1"/>

The first part of the for clause selects each item from the order, and the second part selects the matching product element from the catalog.xml document, using a predicate to identify the one whose number matches the item's num attribute. Another way to accomplish the same thing is by using a where clause instead of a predicate, as shown in Example 6-14. This query yields the same results.

Example 6-14. Two-way join in a where clause

for $item in doc("order.xml")//item, $product in doc("catalog.xml")//product where $item/@num = $product/number return <item num="{$item/@num}" name="{$product/name}" ...