MySQL_约束_修改表时删除约束 —— “更新”

#1 删除非空约束
ALTER TABLE stuinfo
MODIFY COLUMN stuname VARCHAR(20) NULL;

#2 删除默认约束
ALTER TABLE stuinfo
MODIFY COLUMN age INT;

#3 删除主键
ALTER TABLE stuinfo
DROP PRIMARY KEY;

#4 删除唯一
ALTER TABLE stuinfo
DROP INDEX;

#5 删除外键
ALTER TABLE stuinfo
DROP FOREIGN KEY fk_stuinfo_major;