Did you check the User group of functions? If your external database's user id matches the id in Tableau then you could do something like:
USERNAME() = [externalid]
Then just filter for that calculation to be true. Unfortunately many cases the Id's won't match but I'm hoping you can find a way to map them over.
Thank you for your help.
So, if I want to do something like having Datasource as SQL:
"SELECT INFORMATION FROM TABLE_DATA WHERE ID IN (SELECT CURRENT_ID FROM USER_TABLE WHERE USERNAME = USERNAME())" is that correct?
It shouldn't need any custom SQL. Assuming the ID's are the same.
1) Initially build the report so all rows for all users are visible.
2) Create a calculated field using the USERNAME() function. Something like Username() = [databasesusername] <--I'm not sure what your database's column is called.
3) Create a filter on 2) so that it only displays TRUE.