ORACLE - How to convert string to date (with mask)

select to_char( to_date( '20180105', 'yyyymmdd' ), 'dd.mm.yyyy' ) from dual;

Output:

TO_CHAR(TO
----------
05.01.2018

ORACLE - How convert (current) datetime to formated string

select to_char( sysdate, 'dd.mm.yyyy hh24:mi' ) from dual;

Output:

TO_CHAR(SYSDATE,
----------------
17.01.2018 13:26

ORACLE - How get sessionid variable

Oracle sessionid identify user session - every user has own sessionid.

procedure delete_piece
is
  sID number;
begin
  sID := UserEnv( 'SESSIONID' );

  delete from piece
  where 
  sessionid = sID or 
  to_char( dat_net, 'YYYYMMDD' ) <> to_char( sysdate, 'YYYYMMDD' );

  commit;
end;