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【单选题】
scores 表设计如下,查询每个学生的平均成绩的正确 SQL 语句是() 列名 中文解释 数据类型 长度 约束 cno 课程编号 Char 3 主键,外键 sno 学号 Char 9 主键,外键 Score 成绩 numberic 5,2 非空A.
select avg （ Score ） as average from scores;
B.
select Score as average from scores;
C.
select cno , avg （ Score ） as average from scores group by sno
D.
select sno , avg （ Score ） as average from scores group by sno
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