do I use an existing field as the link to child table?
for example I have a list of patients with their bed number. I have a table of bed #'s with with details about the bed (Room #, Floor #, Dept etc). When I create the relationship it adds another "link to ..." field. I just want to use the existing (unique) bed number as my link.
That is a great question. When you establish that link between your two tables, the system will automatically create a new link-to-parent field on the child table. Unfortunately, there is currently no way to "convert" one of your existing fields into the link-to-parent field.
You do, however, have the ability to export the values from the field you would like to "convert" and reimport those values into your new link-to-parent field. In your circumstance, I believe you could export the Bed # field and reimport those values into your new link-to-parent field. You can find more information on importing on our Knowledge Base article in case you are unfamiliar with the process: http://help.trackvia.com/knowledgebase/articles/371965-how-to-import-data