Вы можете попробовать как ниже
SELECT case when Grade<8 then NULL else Name end As Name ,Grade,Marks
FROM Grade,Students
WHERE Marks>=Min_Mark and Marks<=Max_Mark
ORDER BY Grade DESC, Name ASC;
, но я предпочитаю явное объединение, как показано ниже
SELECT case when Grade<8 then NULL else Name end As Name ,Grade,Marks
FROM Grade join Students
on Grade.Marks>=Students.Min_Mark and
Grade.Marks<=Students.Max_Mark
ORDER BY Grade DESC, Name ASC;