如下：创建 CDB 用户则需要加 C##，PDB 用户则不需要。

Connected to:
Oracle Database 19c Enterprise Edition Release 19.0.0.0.0 - Production
Version 19.3.0.0.0

SQL> SQL> 
SQL> 
SQL> 
SQL> show pdbs 

    CON_ID CON_NAME                       OPEN MODE  RESTRICTED
---------- ------------------------------ ---------- ----------
         2 PDB$SEED                       READ ONLY  NO
         3 PDB1                           READ WRITE NO
         4 PDB2                           READ WRITE NO

SQL> 
SQL> create user cdb identified by cdb;
create user cdb identified by cdb
            *
ERROR at line 1:
ORA-65096: invalid common user or role name


Elapsed: 00:00:01.49
SQL> create user c##cdb identified by cdb;

User created.

Elapsed: 00:00:11.09
SQL> alter session set container=pdb1;

Session altered.

Elapsed: 00:00:00.02
SQL> create user cdb1 identified by cdb1;

User created.

Elapsed: 00:00:04.76
SQL> show dbs 
SP2-0158: unknown SHOW option "dbs"
SQL> show pdbs

    CON_ID CON_NAME                       OPEN MODE  RESTRICTED
---------- ------------------------------ ---------- ----------
         3 PDB1                           READ WRITE NO
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