Give the grade score for Rik Unsworth in "Computer Network".
Academic grades awarded for participation in a course are A, B, C, D and F where Grade 'A' means excellent, Grade 'B' means good, Grade 'C' means fair, Grade 'D' means poorly pass, if grade is null or empty, it means that this student fails to pass this course in which grade = NULL;
SELECT CASE grade WHEN 'A' THEN 4 WHEN 'B' THEN 3 WHEN 'C' THEN 2 ELSE 1 END AS result FROM registration WHERE student_id IN ( SELECT student_id FROM student WHERE f_name = 'Rik' AND l_name = 'Unsworth' AND course_id IN ( SELECT course_id FROM course WHERE name = 'Computer Network' ) )
database name:db_id cs_semester
course table course Cols: course id dtype integer, name dtype text, credit dtype integer, difficulty dtype integer, 
prof table prof Cols: professor id dtype integer, gender dtype text, first name dtype text, last name dtype text, email dtype text, popularity dtype integer, teaching ability dtype integer, graduate from dtype text, 
RA table RA Cols: student id dtype integer, capability dtype integer, professor id dtype integer, salary dtype text, 
registration table registration Cols: course id dtype integer, student id dtype integer, grade dtype text, satisfying degree dtype integer, 
student table student Cols: student id dtype integer, first name dtype text, last name dtype text, phone number dtype text, email dtype text, intelligence dtype integer, graduate point average dtype real, type dtype text, 
