-- Create table
create table DEPT_TEST
(
  DEPTNO NUMBER(2) not null,
  DNAME  VARCHAR2(14),
  LOC    VARCHAR2(13)
);
alter table DEPT_TEST
  add constraint PK_DEPT_TEST primary key (DEPTNO);
  
-- Create table
create table EMP_TEST
(
  EMPNO    VARCHAR2(10),
  ENAME    VARCHAR2(10),
  SEX      VARCHAR2(9),
  DEPTNO   NUMBER(2)
);
alter table EMP_TEST
  add constraint FK_DEPTNO_TEST foreign key (DEPTNO)
  references DEPT_TEST (DEPTNO);

SQL>  select * from dept_test;
 
DEPTNO DNAME          LOC
------ -------------- -------------
SQL>   --插入deptno空的时候可以正常插入
SQL>   insert into EMP_TEST values('e005','kk','f',NULL);
 
1 row inserted
SQL>   COMMIT;
 
Commit complete
SQL>   SELECT * FROM EMP_TEST;
 
EMPNO      ENAME      SEX       DEPTNO
---------- ---------- --------- ------
e005       kk         f         
SQL>   insert into EMP_TEST values('e005','kk','f',10);
 
insert into EMP_TEST values('e005','kk','f',10)
 
ORA-02291: integrity constraint (SCOTT_TEST.FK_DEPTNO_TEST) violated - parent key not found
SQL>   COMMIT;
 
Commit complete
SQL>   SELECT * FROM EMP_TEST;
 
EMPNO      ENAME      SEX       DEPTNO
---------- ---------- --------- ------
e005       kk         f