How to use PL/SQL CASE?

CASE expression look like switch statements as in programming languages.

Example: Categorize students according to their exam grades.

SELECT 
studentName, 
CASE 
WHEN studentGrade < 50 THEN 'Weak' 
WHEN studentGrade < 90 THEN 'Good'
WHEN studentGrade < 100 THEN 'Well'
END FROM studentGrades