SQL> CREATE OR REPLACE FUNCTION generate_string_1
  2  RETURN VARCHAR2
  3  IS
  4      l_string        VARCHAR2(35);
  5  BEGIN
  6      SELECT vs.username
  7      ||     TO_CHAR(MOD(ABS(DBMS_RANDOM.RANDOM),10000),'fm0000')
  8      INTO   l_string
  9      FROM   v$session   vs
 10      WHERE  vs.audsid = USERENV('SESSIONID');
 11      
 12      RETURN (l_string);
 13  END generate_string_1;
 14  /
  
Function created.
  
SQL> CREATE OR REPLACE FUNCTION generate_string_2
  2  RETURN VARCHAR2
  3  IS
  4      l_string        VARCHAR2(35);
  5  BEGIN
  6      SELECT vs.username
  7      ||     TO_CHAR(SYSDATE,'YYYYMMDD')
  8      INTO   l_string
  9      FROM   v$session   vs
 10      WHERE  vs.audsid = USERENV('SESSIONID');
 11      
 12      RETURN (l_string);
 13  END generate_string_2;
 14  /
  
Function created.
  
SQL> SELECT generate_string_1 FROM DUAL;
  
GENERATE_STRING_1
-------------------------------------------------------
AMETZER<b>2264</b>
  
SQL> SELECT generate_string_1 FROM DUAL;
  
GENERATE_STRING_1
-------------------------------------------------------
AMETZER<b>0878</b>
  
SQL> SELECT generate_string_2 FROM DUAL;
  
GENERATE_STRING_2
-------------------------------------------------------
AMETZER20030122
  
SQL>