Create table 选课
学号# char(8),
课程编号# char(4),
成绩 smallint,
primary key (学号#, 课程编号#),
foreign key (学号#) references 学生(学号#),
foreign key (课程编号#) references 课程(课程编号#),
check ((成绩 is null ) or (成绩 between 0 and 100 )),