有一个[学生课程]数据库，数据库中包括三个表：
1
create table student(
sno varchar(5) not null primary key,
sname varchar(10),
ssex char(2),
sage int,
sdept varchar(20))
2
alter table student add scome datetime
3
select sno,grade from sg where cno=5 order by grade desc
4
select max(grade) as 最高成绩,avg(grade) as 平均成绩 from sg where cno=1
5
delete from student where sno=05019