Zacznij od uzyskania sum dla każdego z osobna:
# Sum of lakes
SELECT code, count(*) AS sum FROM lakes GROUP BY code
# Sum of mountains
SELECT code, count(*) AS sum FROM mountains GROUP BY code
Następnie połącz wyniki razem i zaznacz wszystkie wiersze, w których suma gór dla kraju jest mniejsza niż liczba jezior:
SELECT l.code AS code, l.sum AS lake_count, m.sum AS mountain_count
FROM
(SELECT code, count(*) AS sum FROM lakes GROUP BY code) AS l JOIN
(SELECT code, count(*) AS sum FROM mountains GROUP BY code) AS m
ON l.code = m.code
WHERE m.sum < l.sum