I have had a 1 to many relationship between course and instructor, which I wanted to drop. When I tried to drop the instructorID in course table it told me that. I couldn\'t
After you run SHOW CREATE table course; you should find the fk symbol which is commonly like the one bellow:
SHOW CREATE table course;
(course_ibfk_1)
it may differ according to your mysql version you are using then drop the foreign key using the fk symbol as follow :
alter table course drop foreign key course_ibfk_1;