几款板子手机电脑跑分结果# PDA - 掌中宝
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stu_name, class_id, time, grade
A 101 1 90
B 101 1 80
A 101 2 100
A 102 1 80
B 102 1 50
B 102 2 70
time 是第几次take这门课, assume一门课只能retake一次.
写SQL, 算出每门课average improvement for students.
A 101 1 90
B 101 1 80
A 101 2 100
A 102 1 80
B 102 1 50
B 102 2 70
time 是第几次take这门课, assume一门课只能retake一次.
写SQL, 算出每门课average improvement for students.