Quick Explanation:

One of the major changes from Drupal 6 to Drupal 7 is the database and how it is structured. In Drupal 6, each content type was its own table. Not the case in Drupal 7. In Drupal 7, each field is its own table and the field’s content type is only a column in the field’s table. I found this while trying to write custom queries in Computed Field. I briefly struggled with how I was going to write relational queries if content types are no longer tables, and each field is its own table … until I discovered entity type and entity ID. I had read and heard about them but didn’t quite grasp the difference until now. Each node created in Drupal 7 has its own entity ID. That entity ID is consistent across all your tables (which are fields). Therefore, to compare things, simply set their entity_id equal to one another.

Computed Field Example: