在cdb的模式下：
SQL> show con_name
CON_NAME
------------------------------
CDB$ROOT
SQL> create user hr identified by hr;
create user hr identified by hr
*
ERROR at line 1:
ORA-65096: invalid common user or role name
看一下报错信息：
SQL> !oerr ora 65096
65096, 00000, "invalid common user or role name"
// *Cause: An attempt was made to create a common user or role with a name
// that wass not valid for common users or roles. In addition to
// the usual rules for user and role names, common user and role
// names must start with C## or c## and consist only of ASCII
// characters.
// *Action: Specify a valid common user or role name.
//
当加上C##或c##的时候就可以创建用户了：
SQL> create user c##hr identified by hr;
User created.
在pdb模式下：
SQL> alter session set container=pdb1;
SQL> create user hr1 identified by hr1;
User created.
总结：
官方文档：
In Oracle Database 12c Release 1 (12.1.0.1), the name of acommon user must begin with C## or c## and the name of a localuser must not begin with C## or c##.
而且，在cdb创建的用户是通用用户（common user），会在cdb和pdb中创建该用户。直接在pdb创建的是本地用户（local user），如果想要在cdb用户访问，需要授权。
SQL>grantconnect to cdb用户；
可以看一下在dba_users表里查看创建后的用户：
SQL> select username from dba_users;
并且，pdb中的DEMO、HR、OE、SH、IX、PM、BI、PDBADMIN、SCOTT默认创建的本地用户在cdb是无法查看到的。