One unified virtual tree structure of categories is provided for different categorized directory resources having different category structures but at least partially similar contents. The unified category tree may be multilingual, .i.e., the tree structure and the unified category descriptions are available in all languages in which the inquiry service is offered to the service users. Further, mapping information is provided which maps the categories of the unified category tree to categories of the different directory resources. A centralized enquiry service system presents at least one category and the respective category description of the unified category structure relevant to an inquiry made by an end user. The user selected one of the unified categories, and the service system performs an inquiry to relevant categories in the target directory resource on the basis of the mapping information and presents resulting data records to the user under the selected unified category.