Sharepoint Designer Form - Autopopulate from Database

Question

I wish to make a form using Sharepoint Designer which will have many fields. Based on the data entered in 1 field, I would like to query an SQL database to autopopulate the other fields. This should be a one-time-job, i.e., when the user exits and returns, he will now be able to see the fields already filled in. Additionally, it would be better if I can store the database login credentials in a separate config file which would be access by Sharepoint to make the connection.