This selects all the even-numbered videos - useful if you are arranging the data in a table. You can use $pos anywhere where you might use the primary variable $v. Its value ranges from 1 to the number of items in the //video sequence. If there are no order by clauses, then the position variables in each for clause follow a nested-loop model as you would expect. If there is an order by clause, the position values represent the position of the items before sorting (which is different from the rule in XSLT).

There are various keywords in the order by clause that give you finer control over how the sorting takes place. The most important is the collation: unfortunately, though, the way collations work is likely to be very product-dependent. The basic idea is that if you are sorting the index at the back of a book, or the names in a phone directory, then you need to apply rather more intelligent rules than simply sorting on the numeric Unicode code value of each character. Upper-case and lower-case variants of letters may need to be treated the same way, and accents on letters have some quite subtle rules in many languages. The working group defining XQuery settled on the simple rule that every collating sequence you might want has a name (specifically, a URI rather like a namespace URI), and it is up to each vendor to decide what collations to provide and how to name them.

Other things you can say in the order specification include defining whether empty values of the sort key (XQuery's equivalent of null values in SQL) should go at the start or end of the sequence, and whether the sort should be stable, in the sense that items with equal sort key values preserve their original order.

Multiple Assignments

One simple syntax note. Instead of writing