根据SQL标准，查询表student（sno,sname,sex,dept)中所有学生的选修课程数，其中选修记录在表SC（s

根据SQL标准，要查询表student中所有年龄age小于所有学生的平均年龄的记录，下面哪条语句适用？()

A select * from student where age< avg(age)

B select * from student having age< avg(age)

C select * from student a where a.age< (select avg(b.age) from student b)

D select * from student a where (select avg(b.age) from student b ) >= a.age

A select dept,avg(age) from student where avg(age)<21

B select dept,avg(age) from student group by dept having avg(age)<21

C select dept,avg(age) from student having avg(age)<21

D select dept,avg(age) from student group by dept where avg(age)<21

A.String sql="Select * from Student where Sname="+txtName.getText();

B.String sql="Select * from Student where Sname='"+txtName.getText()+"'";

C.String sql="Select * from Student where Sname like "+txtName.getText();

D.String sql="Select * from Student where Sname like '"+txtName.getText()+"'";

A.String sql="Select * from Student where Sname="+txtName.getText();

B.String sql="Select * from Student where Sname='"+txtName.getText()+"'";

C.String sql="Select * from Student where Sname like "+txtName.getText();

D.String sql="Select * from Student where Sname like '"+txtName.getText()+"'";

A.String sql="Select * from Student where Sname="+txtName.getText();

B.String sql="Select * from Student where Sname='"+txtName.getText()+"'";

C.String sql="Select * from Student where Sname like "+txtName.getText();

D.String sql="Select * from Student where Sname like '"+txtName.getText()+"'";

A.String sql="Select * from Student where Sname="+txtName.getText();

B.String sql="Select * from Student where Sname='"+txtName.getText()+"'";

C.String sql="Select * from Student where Sname like "+txtName.getText();

D.String sql="Select * from Student where Sname like '"+txtName.getText()+"'";

根据SQL标准，删除表student中对字段sno的唯一性约束，应该使用下面哪条语句？ ()

A drop sno from table student

B alter table student drop sno

C alter table student drop unique(sno)

D alter table student drop sno unique

A delete from student

B delete all from student

C delete * from student

D drop table student

现有学生表,课程表和成绩表，三表中的字段如下：

student表:id,student_name

course表：id,course_name

score表:student_id,course_id,score

前提：每个学生都会参加多门课程的考试，每门课程的成绩在score表中都是一条数据

现若要查询出 平均分在60分以上的学生id和学生姓名以及平均分，以下哪个sql语句是正确的？ ()

A.select student_id,student_name,avg(score) av from student s join score sc on s.id=sc.student_id;

B.select student_id,student_name,avg(score) av from student s join score sc on s.id=sc.student_id where av＞60;

C.select student_id,student_name,avg(score) av from student s join score sc on s.id=sc.student_id where av＞60 group by student_id;

D.select student_id,student_name,avg(score) av from student s join score sc on s.id=sc.student_id group by student_id having av＞60;