Possible query

gistfile1.txt

UPDATED:

-- Obviously select something more useful than favicon_url, which will always be null.
SELECT bid, url, title, visits, hid, i.favicon_url AS favicon_url 
FROM
(
    -- Bookmarks without history.
    SELECT b._id AS bid, b.url AS url, b.title AS title, -1 AS visits, -1 AS hid
    FROM bookmarks b
    WHERE b.folder = 0 AND b.url NOT IN (SELECT url FROM history)

    UNION ALL
    
    -- History with and without bookmark.
    SELECT b._id AS bid, h.url AS url, b.title AS title, h.visits AS visits, h._id AS hid
    FROM history h LEFT OUTER JOIN bookmarks b ON b.url = h.url
    WHERE h.url IS NOT NULL
      AND b.folder IS NULL or b.folder = 0

) LEFT OUTER JOIN images i ON url = i.url_key;




We want to include:

* Folders. These will never be visited.

  SELECT b.id AS bid, b.url AS url, b.title AS title, -1 AS visits, -1 AS hid
  FROM bookmarks b WHERE b.is_folder = 0;

Obviously this will change when we support more types of bookmarks, but you get the idea.

* Bookmarks that have been visited, and those that have not.

  SELECT b.id AS bid, b.url AS url, b.title AS title, h.visits AS visits, h.id AS hid
  FROM bookmarks b LEFT OUTER JOIN history h ON b.url = h.url
  WHERE b.is_folder = 0;

* History items that have been bookmarked, and those that have not.

  SELECT b.id AS bid, b.url AS url, b.title AS title, h.visits AS visits, h.id AS hid
  FROM history h LEFT OUTER JOIN bookmarks b ON b.url = h.url;
  WHERE b.is_folder IS NULL or b.is_folder = 0;


Finally, join these with images:

SELECT bid, url, title, visits, hid, i.favicon_url AS favicon_url 
FROM (
  (
    SELECT b.id AS bid, b.url AS url, b.title AS title, -1 AS visits, -1 AS hid
    FROM bookmarks b WHERE b.is_folder = 0;
  )

    UNION ALL

  (
    SELECT b.id AS bid, b.url AS url, b.title AS title, h.visits AS visits, h.id AS hid
    FROM bookmarks b LEFT OUTER JOIN history h ON b.url = h.url
    WHERE b.is_folder = 0;
  )

    UNION ALL
    
  (
    SELECT b.id AS bid, b.url AS url, b.title AS title, h.visits AS visits, h.id AS hid
    FROM history h LEFT OUTER JOIN bookmarks b ON b.url = h.url;
    WHERE b.is_folder IS NULL or b.is_folder = 0;
  )
) LEFT OUTER JOIN images i ON url = i.url_key;


I haven't tested this at all.