按等级查询

建立一个grade 代表学生的成绩表，并插入数据

create table grade(
low int(3),
upp int(3),
grade char(1)
);
insert into grade values(90,100,'A');
INSERT INTO grade VALUES(80,89,'B');
INSERT INTO grade VALUES(70,79,'C');
INSERT INTO grade VALUES(60,69,'D');
INSERT INTO grade VALUES(0,59,'E');

查询所有同学的s_no,c_no,grade列

select s_no,c_no,grade from score,grade where degree between low and upp;