DECLARE

  --Helper objects
  ERR_TABLE_NOT_EXIST EXCEPTION;
  PRAGMA EXCEPTION_INIT (ERR_TABLE_NOT_EXIST, -00942);

BEGIN

  --Prepare DBMS
  DBMS_OUTPUT.ENABLE;
  DBMS_OUTPUT.PUT_LINE ('-');

  --Store resultset into temporary table
  BEGIN
    insert into TMP_INACTIVE_CLIENTS (CUSTOMER_ID, GROUP_NAME)
    select C.CUSTOMER_ID, G.NAME as GROUP_NAME
    from ...;
  EXCEPTION
    when ERR_TABLE_NOT_EXIST then
      BEGIN
        execute immediate('create global temporary table TMP_INACTIVE_CLIENTS (CUSTOMER_ID number not null, GROUP_NAME varchar2(100)) on commit delete rows');
        execute immediate('alter table TMP_INACTIVE_CLIENTS add (constraint TMP_INACTIVE_CLIENT_PK primary key (CUSTOMER_ID))');
        null;
      END;
    when OTHERS then raise;
  END;

<<done>>
open :RECORDSET for
select * from TMP_INACTIVE_CLIENTS
;

END;
/