问题描述：给表创建外键报错ORA-00906，具体过程如下：
数据库：oracle 11.2.0.4 64位
scott@ORCL 2022-10-14 10:10:40> select index_name,table_name,tablespace_name,status,last_analyzed from user_indexes;

INDEX_NAME      TABLE_NAME      TABLESPACE_NAME STATUS   LAST_ANALYZED
--------------- --------------- --------------- -------- -------------------
PK_EMP          EMP             USERS           VALID    2022-10-11 22:00:04
PK_DEPT         DEPT            USERS           VALID    2022-10-11 22:00:04

scott@ORCL 2022-10-14 10:10:48> select * from app1_emp;

     EMPNO ENAME      JOB              MGR HIREDATE                   SAL       COMM     DEPTNO
---------- ---------- --------- ---------- ------------------- ---------- ---------- ----------
      7369 SMITH      CLERK           7902 1980-12-17 00:00:00        800                    20
      7499 ALLEN      SALESMAN        7698 1981-02-20 00:00:00       1600        300         30
      7521 WARD       SALESMAN        7698 1981-02-22 00:00:00       1250        500         30
      7566 JONES      MANAGER         7839 1981-04-02 00:00:00       2975                    20
      7654 MARTIN     SALESMAN        7698 1981-09-28 00:00:00       1250       1400         30
      7698 BLAKE      MANAGER         7839 1981-05-01 00:00:00       2850                    30
      7782 CLARK      MANAGER         7839 1981-06-09 00:00:00       2450                    10
      7788 SCOTT      ANALYST         7566 1987-04-19 00:00:00       3000                    20
      7839 KING       PRESIDENT            1981-11-17 00:00:00       5000                    10
      7844 TURNER     SALESMAN        7698 1981-09-08 00:00:00       1500          0         30
      7876 ADAMS      CLERK           7788 1987-05-23 00:00:00       1100                    20
      7900 JAMES      CLERK           7698 1981-12-03 00:00:00        950                    30
      7902 FORD       ANALYST         7566 1981-12-03 00:00:00       3000                    20
      7934 MILLER     CLERK           7782 1982-01-23 00:00:00       1300                    10

14 rows selected.
scott@ORCL 2022-10-14 10:17:45> alter table app2_dept add constraint PK_Test_ID primary key(deptno);
scott@ORCL 2022-10-14 10:51:22> alter table app1_emp add constraint fk_app1_deptno foreign key references app2_dept(deptno);
alter table app1_emp add constraint fk_app1_deptno foreign key references app2_dept(deptno)
                                                               *
ERROR at line 1:
ORA-00906: missing left parenthesis


scott@ORCL 2022-10-14 10:53:27> alter table app1_emp add constraint fk_app1_deptno foreign key (deptno) references app2_dept(deptno);

Table altered.

总结：创建语句不正确导致，修改后成功创建外键.