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

A delete from student

B delete all from student

C delete * from student

D drop table student

A.删除student表

B.删除student字段

C.删除student表中的记录

D.删除student数据库

根据SQL标准，要修改表student中所有学生的年龄age，使之在原值基础上减一，下面哪个语句适用？()

A update student set age = 1

B update student set age = age - 1

C update age = age -1 from student

D update from student where age = age -1

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(sno,cno,grade)中，两表中sno为关联字段。下面哪条语句合适？()

A select sno,count(cno) from SC

B select sno,count(cno) from student

C select a.sno,count(cno) from student a left outer join SC

D select a.sno,count(cno) from SC left outer join student a

删除student表中“平均成绩”字段的正确SQL命令是（）。

A.DELETE TABLE student DELETE COLUMN平均成绩

B.ALTER TABLE student DELETE COLUMN平均成绩

C.ALTER TABLE student DROP COLUMN平均成绩

D.DELETE TABLE student DROP COLUMN平均成绩

A.DROP

B.DELETE

C.UPDATE

D.INSERT

删除student表中“平均成绩”字段的正确SQL命令是（）。

A. DELETE TABLE student DELETE COLUMN平均成绩

B. ALTER TABLE student DELETE COLUMN平均成绩

C. ALTER TABLE student DROP COLUMN平均成绩

D. DELETE TABLE student DROP COLUMN平均成绩

A.DELETE student ,where id=11;

B.DELETE FROM student where id=11;

C.DELETE INTO student where id=11;

D.DELETE student where id=11;

A.DROP

B.DELETE

C.UPDATE

D.INSERT