版本：12.2.0
数据库情况：cdb 1 pdb 1
目的：查询出pdb本地的用户（pdb独有的用户）

这个语句为什么报这个错？
SQL> select username ,con_id,user_id from cdb_users where username is not in (select username from cdb_users where con_id=1);
select username ,con_id,user_id from cdb_users where username is not in (select username from cdb_users where con_id=1)
*
ERROR at line 1:
ORA-00908: missing NULL keyword