Help Needed: WordPress Module Development

Goal

I am looking forward to develop a wordpress plugin which needs to interact with database extensively and may need additional database.

What I have done

I have read the following howtos word to word including a few linked pages from the howtos.

Confusion

I am still a bit confused about the database interaction part because none of the above howtos explain the database interaction clearly. In case you know of some resource(s) which explains the interaction clearly and can help me get going with the plugin development, please share it.

Thanks in advance 🙂

PS : I am comfortable with Drupal module development but finding it a difficult to get started with wordpress module development.