Simplenews is a quick and easy way to set up and manage newsletters on Drupal, but if you add it to a site which already has users, you might want a way to quickly subscribe all your users to your newsletter.

Rather than click through all users and manually subscribe them - which can be a little bit boring if you have thousands of users - you can run a few SQL queries to mass-subscribe everyone in one go.

Each user that is subscribed to ANY newsletter has an entry in the simplenews_subscriptions table, which contains their user id, their email address and an assigned subscription id. In addition to that, a user has a discrete entry in the simplenews_snid_tid table for each separate newsletter they're subscribed to.

The latter basically links the unique ID from the subscriptions table to the term ID from the Newsletter taxonomy.