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# First, delete all subscribers
DELETE wpu FROM wp_users AS wpu
INNER JOIN wp_usermeta AS wpm ON wpu.ID = wpm.user_id
WHERE wpm.meta_key = 'wp_capabilities' AND wpm.meta_value = 'a:1:{s:10:"subscriber";b:1;}';
UPDATE wp_posts SET post_author = 1;

# Then, delete the user meta
DELETE FROM wp_usermeta WHERE user_id NOT IN (SELECT ID FROM wp_users)