Migrating Only DRUPAL7 DATABASE from Microsoft SQL to MYSQL
A possible approach for this is to use the Drupal-to-Drupal data migration module (aka "migrate_d2d") for this. Quote from its project page:


  ... provides a framework based on the Migrate API for migrating content from Drupal 5, Drupal 6, or Drupal 7 sites into the Drupal 7 site where it is installed.