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user [Oracle SQL]

user is a pseudo column. It returns the name of the user currently connected (and hence executing the query) to the session that executes the select statement. 
conn rene/rene
select user from dual;

USER
------------------------------
RENE

conn bi/pw_bi
select user from dual;

USER
------------------------------
BI

alter session set current_schema = rene;
select user from dual;

USER
------------------------------
BI