作业一：SELECT class_id,avg(score) average FROM students GROUP BY class_id;
作业二：SELECT class_id,gender,avg(score) average FROM students GROUP BY class_id,gender ORDER BY class_id,gender;
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木丶丶心
作业一：SELECT class_id,avg(score) average FROM students GROUP BY class_id;
作业二：SELECT class_id,gender,avg(score) average FROM students GROUP BY class_id,gender ORDER BY class_id,gender;