Assign all Post in Wp to a specific Category

1. Find out the ID of the category that you want to assign. You’ll see it in the URL when editing the category (URL will be something like ?taxonomy=category&tag_ID=3&post_type=post, here 3 is the ID)

2. Assuming that the ID is 3, you can now run this query in your database (eg via PHPMyAdmin)

   INSERT IGNORE INTO wp_term_relationships
   (object_id, term_taxonomy_id)
   (  
      SELECT DISTINCT ID, 3
      FROM wp_posts
      WHERE post_type="post" AND post_status="publish"
   );
   

If you use a different prefix than wp_, be sure to change it. Also change the 3 to the ID you want.