无法添加或更新子行：外键约束失败（`sb101db`.`students`,

Question

MySQL> create table students(
  S_ID varchar(10),
  S_NAME varchar(20),
  S_ADDRESS varchar(50) DEFAULT "BIHAR",
  S_PHONE varchar(13) NOT NULL UNIQUE,
  S_AGE int , 
  foreign key (S_ID) references course (C_ID) on delete set null
);

MySQL insert into students values("S1", "ram", "Delhi", "2663428764", 30);

ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`sb101db`.`students`, CONSTRAINT `students_ibfk_1` FOREIGN KEY (`S_ID`) REFERENCES `course` (`C_ID`) ON DELETE SET NULL)