gistfile1.sql

select tag.*, count(*) as count from tag 
  LEFT OUTER JOIN category_tag ON tag.name_encoded = category_tag.name_encoded 
  WHERE category_tag.name_encoded IS NULL 
  GROUP BY tag.name_encoded
  ORDER BY count DESC
  LIMIT 1;

SELECT tag.*, count(*) as count FROM category_tag.name_encoded
  JOIN tag ON tag.name_encoded = category_tag.name_encoded
  WHERE category_tag.name_encoded IS NULL
  GROUP BY tag.name_encoded
  ORDER BY count DESC;