I have added a new (hidden) attribute to an existing subscriber database. I now want to populate that attribute/field with data that I have in a delimited text file, without changing all other data associated with the other attributes for each subscriber in the database. I want to add this data retrospectively to existing subscribers using a file that I have created on my local machine, without changing any of the other existing data for each subscriber. I also only want to populate this attribute for a subset of my whole subscriber list, and some of the subscribers will be on multiple lists - I add this only for information, not because I believe this will cause an issue.
I have searched but am not confident I have found the right way of doing this. Any help gratefully received.