T24 function/query to select distinct records

Depending on your needs you can use the SAVING UNIQUE clause, e.g.

SELECT FBNK.CUSTOMER SAVING UNIQUE RESIDENCE BY RESIDENCE
SAVE-LIST ResidenceCountry


SAVING
Use with SELECT, SSELECT to create a list containing field values instead of record IDs. If you have field values that are also record IDs in another file, this is useful. The syntax in SELECT or SSELECT sentences is as follows:
SAVING [ UNIQUE ] field
The UNIQUE keyword omits duplicate values in the saved field from the select list.
If you specify multiple SAVING clauses in a SELECT or SSELECT sentence, the entire value of each field becomes an element in the select list. Value marks separate multiple values, subvalue marks separate subvalues.

You can use it e.g. in a BUILD.ROUTINE that returns an active selectionlist to the enquiry.