How many students, who have a GPA between 3 to 4, failed a course?
GPA is an abbreviated name of Grade Point Average where GPA between 3 to 4 refers to gpa BETWEEN 3 AND 4; If grade is null or empty, it means that this student fails to pass this course;
SELECT COUNT(T2.student_id) FROM student AS T1 INNER JOIN registration AS T2 ON T1.student_id = T2.student_id WHERE T2.grade IS NULL AND T1.gpa BETWEEN 3 AND 4