alter table score drop grades
alter table score add grades nvarchar(10) null
update score set grades = '优' where score.final >=90
update score set grades = '良' where score.final >=80 and score.final <90
update score set grades = '中' where score.final >=70 and score.final <80
update score set grades = '及格' where score.final >=60 and score.final <70
update score set grades = '不及格' where score.final <60
select * from student,score where student.studentno = score.studentno
