--建立两个表
create table publishinfo
(
publishid int,--出版社ID
publish varchar2(20),--出版社
constraint un_publish unique (publish)
);
create table bookinfo
(
bookid int,--图书ID
loan char(1),--借出状态
publish varchar2(20),--出版社
constraint fk_publish foreign key (publish)
references publishinfo (publish)
on delete cascade
)；
--开始添加数据
insert into publishinfo
(publishid ,publish)
values
(10000,111);
insert into publishinfo
(publishid ,publish)
values
(10000,null);
insert into bookinfo
(publishid ,publish)
values
(10000,222);--依旧会进行验证

--首先你的不是组合fk,组合是两个或两个以上联合作为外键，如下id,sid在一起叫组合fk
create table t(
id int,
sid int,
name varchar2(20),
constraint pk_t primary key(id,sid)
);
create table ct(
cid int,
id int,
sid int,
name varchar(20),
constraint pk_ct primary key(cid),
constraint fk_ct_t_id_sid foreign key(id,sid) 
references t(id,sid)
);
insert into t values(1,1, 'name1');
insert into ct values(1,1,1, 'n1 ');
--此处可以成功，就像你说的：则将跳过组成该约束的所有值的验证。
insert into ct values(2,1,null, 'n2');
--此处失败
insert into ct values(2,1,2, 'n2');

------解决方案--------------------
FOREIGN KEY约束可以包含空值

任何组合FOREIGN KEY约束的列包含空值