插入数据的时候提示外码约束失败怎么办？

Question

create table student
(Sno char(20)primary key not null,
Sname char(6)not null,
Ssex char(2)null,
Sage int null,
Sdept char(30) null);

create table course
(Cno char(3)primary key not null,
Cname char(20)not null,
Cpno char(3),
Ccredit int not null,
foreign key (Cpno)references course(Cno));

create table SC
(Sno char(20)not null,
Cno char(3)not null,
grade float null,
foreign key (Sno)references student(Sno),
foreign key (Cno)references course(Cno));

insert into student
(Sno,Sname,Ssex,Sage,Sdept)
values
("200215121","李勇","男","20","CS"),
("200215122","刘晨","女","19","CS"),
("200215123","王敏","女","18","MA"),
("200515125","张立","男","19","IS");
insert into course
(Cno,Cname,Cpno,Ccredit)
values
("1","数据库","5","4"),
("2","数学","1","2"),
("3","信息系统","6","4"),
("4","操作系统","7","3"),
("5","数据结构","6","4"),
("6","数据处理",null,"2"),
("7","PASCAL语言",null,"4");

insert into SC
(Sno,Cno,grade)
values
("200215121","1","92"),
("200215121","2","85"),
("200215121","3","88"),
("200215122","2","90"),
("200215122","3","80");

Answer 1

你这说的也不清楚啊？！
这数据我用pgsql测试是没有报错的。
一般外键约束失败的原因就是引用的主表需要没有数据