The USER function and the SYSTEM_USER function are identical and they return the user name together with the host name.
The CURRENT_USER, USER with a similar feature returns the user names who has logged on to the current database as character strings.
USER()
SYSTEM_USER()
--selecting the current user on the session
SELECT USER;
CURRENT_USER
======================
'PUBLIC'
SELECT USER(), CURRENT_USER;
user() CURRENT_USER
============================================
'PUBLIC@cdbs006.cub' 'PUBLIC'
--selecting all users of the current database from the system table
SELECT name, id, password FROM db_user;
name id password
=========================================================
'DBA' NULL NULL
'PUBLIC' NULL NULL
'SELECT_ONLY_USER' NULL db_password
'ALMOST_DBA_USER' NULL db_password
'SELECT_ONLY_USER2' NULL NULL