Hi Accesswang,
I believe the only way to submit data to a database using standard InfoPath functionality is to create a form with a Main DOM schema bound to the database. To do this, create a new form and choose "From Database schema" - this is what allows submission to a database. You should be able to achieve the same things that you're doing with the seconday dom on this new Main DOM - just different XPaths.
On the other hand, you could try our Database Accelerator product which provides much more robust database submission through web services. The web services will also read from the database so that you don't have to store the SQL password in the form. Check out the free developer download here:
http://www.qdabra.com/proddetail.asp?prod=QDBXL1
Matt Faus / Microsoft InfoPath MVPQdabra® Software / Streamline data gathering to turn process into knowledge