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key('programs', xs:dateTime('2001-07-05T19:30:00')) will return all the programs that start at 19:30 on 5 July 2001, but key('programs', '2001-07-05T19:30:00') will return nothing. The fact that the types have to match is particularly relevant if you pass a node as the second argument of the key() function. In this case, the untyped value of the node is implicitly cast to a string, and thus you will only get back values if the key value is a string as well. So key('programs', Start) (which would otherwise give you all the programs that started at the same time as this one) won t give you anything either. You need to explicitly cast the value of the <Start> element, as in the following: key('programs', xs:dateTime(Start))

If a key value is anything other than a string, remember to cast the second argument of the key() Tip

As you ll remember from 4, when we looked at comparing strings, and from the last chapter, when we looked at sorting, there s no fixed way of comparing two strings: for example, you might want to compare them in a case-insensitive way or based on a particular language. If you need to, you can specify the collation used for comparing strings used as key values using the collation attribute on <xsl:key>.

Summary You can specify the key value used to index an element either with the use attribute (which contains an XPath expression) or with the sequence constructor content of the <xsl:key> element. The types of key values are significant. The collation used to compare key values that are strings is specified with the collation attribute.

Keys are not bound by the limitation on IDs that states there can only be one element with a particular identifier. The same key value can be used to access multiple elements. This is particularly useful when you want to look at reverse relationships (such as those used by the idref() function), from an element to all the elements that refer to it. For example, you could set up a key that would allow you to find all the programs that belong to a particular series. The elements that you want to retrieve are <Program> elements, so the match pattern needs to match them, and the key values that they need to be indexed by are the values of their <Series> elements. The key definition looks like this: <xsl:key name="programsBySeries" match="Program" use="Series" />

It doesn t matter that several programs belong to the same series all of them are returned by the call to the key() function. For example, the following returns all <Program> elements that are part of the EastEnders series: key('programsBySeries', 'EastEnders')

Note When you use the key() function, the second argument must match the key value exactly. There s no way to retrieve all the Star Trek episodes (those programs whose series starts with 'StarTrek') using this key, for example, because the programs are indexed by the entirety of their series name.

What s more, while usually the key value is a single item, you can also use an expression that evaluates to a sequence. If you do, then the indexed node can be retrieved using the value of any of the items in the sequence, essentially giving the element multiple identifiers. This is particularly useful when you want to access the same element in many different ways. For example, to enable us to retrieve all the <Program> elements that star a particular actor, we could set up a key that again matches <Program> elements but this time using the <Name> child of the <Actor> child of the <CastMember> child of the <CastList> element, as follows: <xsl:key name="programsByActors" match="Program" use="CastList/CastMember/Actor/Name" /> When it s evaluated from the context of a <Program> element, the path CastList/CastMember/ Actor/Name returns a sequence containing several <Name> elements (one for each actor in the program). The values of these <Name> elements are used to index the <Program> element within the programsByActors key. Whichever actor is named by the call to the key, the <Program> element will be returned by it. For example, given the following cast list as in TVGuide4.xml: <Program> ... <CastList> <CastMember> <Character><Name>Zoe Slater</Name>...</Character> <Actor><Name>Michelle Ryan</Name>...</Actor> </CastMember> <CastMember> <Character><Name>Jamie Mitchell</Name>...</Character> <Actor><Name>Jack Ryder</Name>...</Actor> </CastMember> <CastMember> <Character><Name>Sonia Jackson</Name>...</Character> <Actor><Name>Natalie Cassidy</Name>...</Actor> </CastMember> ... </CastList> ... </Program>