SQL中增添列名显示列名无效

Question

--增加等级列level，初始级别均为D
ALTER TABLE student1
Add level char(2)
UPDATE student1
SET level='D'
--等级制
--A:90100
update student1
SET level = 'A'
where score >= 90
--B:7090
update student1
set level = 'B'
where score >= 70 and score < 90
--C:60~70
update student1
set level = 'C' where score >= 60 and score < 70
--D:<60
update student1
set level= 'D'
where score<60