SQL CLR aggregate parameters

Here is an example of using a SqlUserDefinedAggregate: http://msdn.microsoft.com/en-us/library/91e6taax(v=vs.80).aspx

Which allows you to do:

SELECT LastName, COUNT(LastName) AS CountOfLastName, dbo.CountVowels(LastName) AS CountOfVowels
FROM Person.Conta开发者_JAVA技巧ct
GROUP BY LastName
ORDER BY LastName

How could I convert this to dbo.CountLetters(LastName, 'listOfLetters')? In other words, how can I take an extra parameter when aggregating values? This is easy with a regular CLR function, but how to do it here escapes me. Thanks!

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Depends on what version you're running. For SQL Server 2005 you can't as the input is restricted to a single parameter. For 2008, see this article for an example.