This chapter explains how to sort and group data from input documents. It covers sorting in FLWORs, grouping results together, and calculating summary values using the aggregation functions.
Path expressions, which are most often used to select elements and attributes from input documents, always return items in document order. FLWORs by default return results based on the order of the sequence specified in the
for clause, which is also often document order if a path expression was used.
The only way to sort data in an order other than document order is by using the
order by clause of the FLWOR. Therefore, in some cases it is necessary to use a FLWOR where it would not otherwise be necessary. For example, if you simply want to select all of your items from an order, you can use the path expression
doc("order.xml")//item. However, if you want to sort those items based on their
num attribute, you have to use a FLWOR.
Example 7-1 shows an
order by clause in a FLWOR.
Example 7-1. The order by clause
for $item in doc("order.xml")//item order by $item/@num return $item
The results will be sorted by item number. The syntax of an
order by clause is shown in Figure 7-1.
order by clause is made up of one or more ordering specifications, separated by commas, each of which consists of an expression and an optional modifier. The expression can only return one value for each item being sorted. In Example 7-1, there is only ...