Globally enable comments for all users
UPDATE wp_posts SET comment_status = 'open';
Globally disable comments for all users
UPDATE wp_posts SET comment_status = 'closed';
Globally enable comments for registered users only
UPDATE wp_posts SET comment_status = 'registered_only';
Globally enable/disable comments before a certain date
For this query, specify the
comment_status as either
registered_only. Also, specify the date by editing the
2008-01-01 to suit your needs.
UPDATE wp_posts SET comment_status = 'closed' WHERE post_date < '2008-01-01' AND post_status = 'publish';
This last query is one that I will be using a few times each year (or as often as I can remember it) to disable comments on old posts. Ultimately, I will combine this query with a similar one (provided below) for pingbacks and trackbacks to manage discussion options with a single step.