Oracle: database not showed I created an Oracle's container database (CDB) at CentOS named demo0, then I also created an user named c##demo.  

The issue is that the dabase is not showed at using ORACLE_SID=demo0 but it's only showed with ORACLE_SID=ORCLCDB:

[oracle@centos-8 ~]$ echo $ORACLE_SID
ORCLCDB
[oracle@centos-8 ~]$ sqlplus sys as sysdba

SQL*Plus: Release 18.0.0.0.0 - Production on Sat Jun 13 09:00:43 2020
Version 18.3.0.0.0

Copyright (c) 1982, 2018, Oracle.  All rights reserved.

Enter password: 

Connected to:
Oracle Database 18c Enterprise Edition Release 18.0.0.0.0 - Production
Version 18.3.0.0.0

SQL&gt; select name, open_mode from v$pdbs;

NAME
--------------------------------------------------------------------------------
OPEN_MODE
----------
PDB$SEED
READ ONLY

ORCLPDB1
MOUNTED

DEMO0
MOUNTED

SQL&gt; Disconnected from Oracle Database 18c Enterprise Edition Release 18.0.0.0.0 - Production
Version 18.3.0.0.0


[oracle@centos-8 ~]$ export ORACLE_SID=demo0
[oracle@centos-8 ~]$ echo $ORACLE_SID
demo0
[oracle@centos-8 ~]$ sqlplus sys as sysdba

SQL*Plus: Release 18.0.0.0.0 - Production on Sat Jun 13 09:01:24 2020
Version 18.3.0.0.0

Copyright (c) 1982, 2018, Oracle.  All rights reserved.

Enter password: 

Connected to:
Oracle Database 18c Enterprise Edition Release 18.0.0.0.0 - Production
Version 18.3.0.0.0

SQL&gt; select name, open_mode from v$pdbs;

NAME
--------------------------------------------------------------------------------
OPEN_MODE
----------
PDB$SEED
READ ONLY
ORCLCDB and demo0 are 2 seperate instances based on your output..

You have an empty CDB called demo0 with the seed PDB only.

And you have another CDB called ORCLCDB, in which you created a PDB called DEMO0.

Of course DEMO0 PDB is not listed in demo0 CDB, because the DEMO0 PDB was created in the ORCLCDB CDB.