에러메세지 error code: 1822. failed to add the foreign key constraint 참고 https://stackoverflow.com/questions/43511183/mysql-error-1822-failed-to-add-foreign-key-constraint-missing-index-for-contra MySQL Error 1822: Failed to add foreign key constraint; missing index for contraint BUT index exists I am trying to add an foreign key to my flightschedule table but it fails, but I do not really know why. ..