-- select URL entities, not yet downloaded
-- entity id + lowest pubDate, pubTime of its' instances
SELECT
e.id,
e.name,
d.pubDate,
d.pubTime
FROM entities e, instances i, documents d
WHERE
e.id=(
    SELECT ee.id
    FROM entities ee
    WHERE ee.enttype_id=9
        AND NOT EXISTS (SELECT * FROM downloadurl WHERE entity_id=ee.id)
    LIMIT 1
    )
AND i.entity_id=e.id -- JOIN
AND i.item_id=d.id   -- JOIN
ORDER BY (d.pubDate + d.pubTime)
LIMIT 1;