I have a form with an existing data source. This data source has a one to many relationship to another table that is not an existing data source. Even though this second table contains multiple records (one to many), the field in the table that I want is duplicated across all records. Therefore I want to add this second table as a data source, but only return one record from it.
If I add the second table directly, than my form contains a line for each record instead of just one.
This problem was solved by creating a view to use as the new datasource. This view defined a calculated column that was based on a method that contained a query string that used TOP 1. The details in much more detail are at Martin Dráb's blog: https://community.dynamics.com/ax/b/goshoom/archive/2015/06/29/join-first-line-in-ax-2012.