SELECT *, 
    IF(points>=90, "Brilliant", IF(points>=80, "Gold", IF(points>=60, "Silver", "Lazy"))) AS class
FROM `students`
ORDER BY points DESC

UNION

SELECT *, 'Brilliant' AS class
FROM `students`
WHERE points >= 90
UNION
SELECT *, 'Gold'
FROM `students`
WHERE points >= 80 
    AND points < 90
UNION
SELECT *, 'Silver'
FROM `students`
WHERE points >= 60 
    AND points < 80
UNION
SELECT *, 'Lazy'
FROM `students`
WHERE points < 60
ORDER BY points DESC

CASE WHEN

SELECT *, 
    CASE 
      	WHEN points>90 THEN "Brilliant"
      	WHEN points>80 THEN "Gold"
      	WHEN points>60 THEN "Silver"
      	ELSE "Lazy"
    END as 'class'
FROM `students`
ORDER BY points DESC