mysql如何在删除外键之前判断外键是否存在

如题，怎样先判断外键是否存在，然后在删除，像
if EXISTS((SELECT COUNT(*) FROM information_schema.KEY_COLUMN_USAGE where constraint_name='fk_emp_dept_id') > 0)
alter table org_employee drop foreign key fk_emp_dept_id;
还有，
if EXISTS((SELECT COUNT(*) FROM information_schema.KEY_COLUMN_USAGE where constraint_name='fk_emp_dept_id') > 0)
BEGIN
alter table org_employee drop foreign key fk_emp_dept_id;
END
都会报错，求大神解答