1) SELECT t1.MF_ID, count(*)
FROM `производители` t1
LEFT JOIN `продукты` t2 ON t1.MF_ID = t2.PD_MFID
GROUP BY t1.MF_ID
HAVING count(*)=0
2) SELECT t1.MF_ID, max(t2.PD_PRICE)
FROM `производители` t1
LEFT JOIN `продукты` t2 ON t1.MF_ID = t2.PD_MFID
GROUP BY t1.MF_ID
ORDER BY max(t2.PD_PRICE) DESC
3) SELECT t1.MF_ID, count(*)
FROM `производители` t1
LEFT JOIN `продукты` t2 ON t1.MF_ID = t2.PD_MFID
GROUP BY t1.MF_ID
ORDER BY count(*) DESC
LIMIT 2,1