How to cadcade filter comboboxes based on an SQL query (data item list) rather than map services?

Using a map service in the query task yields a whole table of records from which to query, filtering by previous combobox values. However when using a data list as an input (populated from a SQL Source query previously), there is only one list/field so I can't figure out how to filter by previous selections.

Has anyone done this?

thanks!