SQL> select job,sum(comm) from emp where 1=1 group by job;

JOB        SUM(COMM)
--------- ----------
CLERK
SALESMAN        2200
PRESIDENT
MANAGER
ANALYST

SQL> select job,sum(comm) from emp where 1=1 and comm is not null group by job;

JOB        SUM(COMM)
--------- ----------
SALESMAN        2200

SQL>

SQL> select deptno,sum(comm) from emp where 1=1 group by deptno having sum(comm) is not null;

    DEPTNO  SUM(COMM)
---------- ----------
        30       2200

SQL>

SQL> select 1 from dual where sysdate BETWEEN '2017-07-01 00:00:00' AND '2017-09-31 23:59:59'
  2  /
select 1 from dual where sysdate BETWEEN '2017-07-01 00:00:00' AND '2017-09-31 23:59:59'
                                                                   *
ERROR at line 1:
ORA-01861: literal does not match format string

SQL> with t as (
  2  select to_date('26/09/2017 00:00:00','dd.mm.yyyy hh24:mi:ss') d from dual union all
  3  select to_date('26/09/2017 10:00:00','dd.mm.yyyy hh24:mi:ss') d from dual union all
  4  select to_date('26/09/2017 23:59:59','dd.mm.yyyy hh24:mi:ss') d from dual union all
  5  select to_date('27/09/2017 00:00:00','dd.mm.yyyy hh24:mi:ss') d from dual union all
  6  select to_date('27/09/2017 10:00:00','dd.mm.yyyy hh24:mi:ss') d from dual
  7  )
  8  select * from t where d between date '2017-09-26' and date '2017-09-27'
  9  /

D
-------------------
26.09.2017 00:00:00
26.09.2017 10:00:00
26.09.2017 23:59:59
27.09.2017 00:00:00

select * from t where d >=date '2017-09-26' and d<date '2017-09-27'

select date '2017-09-27'-interval '1' second from dual 

  1* select to_date('31/12/2016 23:59:60','dd.mm.yyyy hh24:mi:ss') d from dual
SQL> /
select to_date('31/12/2016 23:59:60','dd.mm.yyyy hh24:mi:ss') d from dual
               *
ERROR at line 1:
ORA-01852: seconds must be between 0 and 59