查询student表中的所有非空email信息, 以下语句正确的是( )。（2.6）
A. Select email from student where email !=null
B. Select email from student where email not is null
C. Select email from student where email <> null
D. Select email from student where email is not null
现有书目表book,包含字段:价格price (float),类别type(char); 现在查询各个类别的平均价格、类别名称,以下语句正确的是( )。（2.6）
A. select avg(price),type from book group by type
B. select count(price),type from book group by price
C. select avg(price),type from book group by price
D. select count (price),type from book group by type
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