Whether a string compare operation is case sensitive or not depends on the collation settings in the database. If you want to override the database collation, you can specify a specific collation after the name of the column. Of course, this only works in a SQL query, not in an aggregate. For example:

SELECT *
FROM {User}
WHERE {User}.[Username] COLLATE SQL_Latin1_General_CP1_CS_AS like 'k%'

only selects usernames starting with a lowercase 'k', while without the COLLATE, it also selects usernames with an uppercase 'K'.