SELECT s.SubjectsKey, CountPerKey = ISNULL(d.CountPerKey,0)
FROM Subjects s
LEFT JOIN (
	SELECT SubjectsIKey, CountPerKey = COUNT(*) 
	FROM Subjects_items
	GROUP BY SubjectsIKey
) d ON d.SubjectsIKey = s.SubjectsKey

SELECT s.SubjectsKey, CountPerKey = COUNT(i.SubjectsIKey)
FROM Subjects s
LEFT JOIN Subjects_items i 
	ON i.SubjectsIKey = s.SubjectsKey
GROUP BY s.SubjectsKey

SELECT s.SubjectsKey, CountPerKey = ISNULL(d.CountPerKey,0)
FROM Subjects s
OUTER APPLY (
	SELECT SubjectsIKey, CountPerKey = COUNT(*) 
	FROM Subjects_items i
	WHERE i.SubjectsIKey = s.SubjectsKey
	GROUP BY SubjectsIKey
) d