CURRENT_USER and USER are used interchangeably. They return the user name that is currently logged in to the database as a string.
USER() and SYSTEM_USER() are used interchangeably. They return the user name with a host name.
CURRENT_USER
USER
--selecting the current user on the session
SELECT USER;
CURRENT_USER
======================
'PUBLIC'
SELECT USER(), CURRENT_USER;
user() CURRENT_USER
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'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
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'DBA' NULL NULL
'PUBLIC' NULL NULL
'SELECT_ONLY_USER' NULL db_password
'ALMOST_DBA_USER' NULL db_password
'SELECT_ONLY_USER2' NULL NULL