A common user is a database user that has the same identity in the root and in every existing and future pluggable database (PDB). Every common user can connect to and perform operations within the root, and within any PDB in which it has privileges.
Every common user is either Oracle-supplied or user-created. Examples of Oracle-supplied common users are SYS and SYSTEM.
Common users have the following characteristics:
A common user can log in to any container (including CDB$ROOT) in which it has the CREATE SESSION privilege.
A common user need not have the same privileges in every container. For example, the c##dba user may have the privilege to create a session in the root and in one PDB, but not to create a session in a different PDB. Because a common user with the appropriate privileges can switch between containers, a common user in the root can administer PDBs.
The name of every user-created common user must begin with the characters c## or C##. (Oracle-supplied common user names do not have this restriction.)
No local user name may begin with the characters c## or C##.
The names of common users must contain only ASCII or EBCDIC characters.
Every common user is uniquely named across all containers.
A common user resides in the root, but must be able to connect to every PDB with the same identity.
The schemas for a common user can differ in each container.
For example, if c##dba is a common user that has privileges on multiple containers, then the c##dba schema in each of these containers may contain different objects.
See Also:
Oracle Database Concepts for more information about common users in a multitenant container database (CDB)
Oracle Database Security Guide to learn about common and local account