ORA-04044: 此处不允许过程, 函数, 程序包或类型？？？？

用Orale代码建表时，出现

SQL> comment on column SCORE.cno
2 is ‘学号（外键）‘;
comment on column SCORE.cno
is ‘学号（外键）‘
ORA-04044: 此处不允许过程, 函数, 程序包或类型
SQL> comment on column SCORE.cname
2 is ‘课程号(外键)‘;
comment on column SCORE.cname
is ‘课程号(外键)‘
ORA-04044: 此处不允许过程, 函数, 程序包或类型
SQL> comment on column SCORE.tno
2 is ‘成绩‘;
comment on column SCORE.tno
is ‘成绩‘
ORA-04044: 此处不允许过程, 函数, 程序包或类型
SQL> alter table SCORE
2 add constraint PK_SCORE primary key (SNO,CNO);
alter table SCORE
add constraint PK_SCORE primary key (SNO,CNO)
ORA-00942: 表或视图不存在