XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (170 page)

Sorting the Groups

If there are any

elements as children of the

, these affect the order in which the groups are processed. They do not affect the order of the items within each group, nor which item in a group is considered to be the initial item.

The
select
expression in an

element calculates a sort key that affects the group as a whole, but it is always evaluated with respect to the initial item in the group. The initial item in a group is the item within the group that was first in population order. The
select
expression is evaluated with this item as the context item, with the position of this item relative to the initial items of other groups as the context position, and with the number of groups as the context size.

If any of the attributes of the

elements are attribute value templates, then the XPath expressions in these attribute value templates are evaluated with the same context item, position, and size as the
select
expression of the containing

element.

If there are no

elements, or in cases where the initial items in two groups have the same values for their sort keys, the groups are processed in order of first appearance; that is, if the initial item of group G appeared in the population before the initial item of group H, then group G is processed before group H.

In practice, if the groups are sorted at all then they are nearly always sorted by the value of the grouping key:
group the addresses in each city, sorting the groups by city
. This can be conveniently coded as:

The functions
current-group()
and
current-grouping-key()
are described on page 739 in Chapter 13.

An

element within

is used to sort the groups. To sort the items within each group, use an

element within the inner

, or write:

The

instruction is described on page 437.

You can also use the

instruction to sort the population before grouping starts.

Usage and Examples

The following sections give a number of examples of how

can be used to solve grouping problems. They are organized according to the four ways of defining the grouping criteria:
group-by
,
group-adjacent
,
group-starting-with
, and
group-ending-with
.

Using group-by

This is by far the most common kind of grouping. We'll start with a simple case.

This general design pattern, where

is used at the outer level to iterate over the groups, and an inner

is used to iterate over the items within a group, is typical. But there are a number of useful variations:

• The inner loop is sometimes better done using
  
  , especially if the group includes elements of different types.
  
• Sometimes the entire inner loop can be written as
  
  , especially when generating XML output.
  
• Sometimes the requirement is not to display the items in each group, but to calculate some aggregate function for these items: for example to list for each department, the name of the department, the number of employees, and the maximum salary. In this case, the inner loop might not be explicit. The number of employees in the group is easily computed as
  count(current-group())
  . Our sample data doesn't show salary, but if this was available as an extra attribute on the
  
  element, then you could easily calculate the maximum salary as
  max(current-group()/@salary)
  .
  
• If the requirement is to eliminate duplicates rather than to group all the items (in our example, to output a list of departments), then the inner loop can be omitted entirely. But in this case it may be simpler to use the
  distinct-values()
  function described on page 749.
  
• If you need to number the groups, or test whether you are processing the last group, then within the
  
  element you can use
  position()
  and
  last()
  in the usual way. At this level, the context item is the initial item of the group being processed, and
  position()
  and
  last()
  refer to the position of this item in a list that contains the initial item of each group, in processing order.
  

The example above was expressed as a grouping problem (“list the employees grouped by department”), so it is easy to see that

can be used in the solution. Sometimes grouping problems are not so easy to recognize. This might be the case if the example above were expressed as “for each department, list the name of the department and the number of employees.”