Need a rolling count of rows from table using just SQL

Question

Oracle 11gR2
RHEL 6.4
Given the following table data:
EMPLOYEE     HIRE_DATE     TERM_DATE     DEPT
John Doe          01/01/13                                   101
Jane Smith       01/05/13                                   102
Bob Jones        02/04/13          04/22/13             102
Jenny Boo        03/12/13          03/31/13             103
Joe Schmoe     03/24/13                                    102
Bill Max            04/23/13                                   103
Jill Clay            04/24/13                                   103
Joe Boom         05/11/13                                   102
I want to return the number of employees that are still employed for each month so long as they were hired anytime in that month and irregardless if they were terminated later that month (i.e only the month concerns me).  So I would be expecting
MONTH  EMPLOYEES
Jan         2
Feb        3
Mar        5
Apr        6   (Jenny Boo has been terminated)
May       6   (Jenny Boo and Bob Jones have been terminated)  
I know there are some bright people out there that are SQL experts but I am not one of them.  If there is a way to do this in just SQL I would like to see it.

Answer 1

Assuming you want count just for months where someone was hired or terminated:
with t as (
            select  trunc(hire_date,'mm') dt,
                    1 weight
              from  emp1
           union all
            select  last_day(term_date) + 1 dt,
                    -1 weight
              from  emp1
              where term_date is not null
select  to_char(dt,'FMMonth, YYYY') month,
        sum(sum(weight)) over(order by dt) employees
  from  t
  group by dt
  order by dt
MONTH            EMPLOYEES
January, 2013            2
February, 2013           3
March, 2013              5
April, 2013              6
May, 2013                6
SQL>
If you want all months within given range:
with r as (
           select  date '2013-03-01' from_dt,
                   date '2013-12-01' to_dt
             from  dual
     t as (
            select  trunc(hire_date,'mm') dt,
                    1 weight,
                    from_dt
              from  emp1,
                    r
              where hire_date <= last_day(to_dt)
           union all
            select  last_day(term_date) + 1 dt,
                    -1 weight,
                    from_dt
              from  emp1,
                    r
               where last_day(term_date) + 1 <= to_dt
           union all
            select  add_months(from_dt,level - 1) dt,
                    0 weight,
                    from_dt
              from  r
              connect by add_months(from_dt,level - 1) <= to_dt
     s as (
           select  dt,
                   sum(sum(weight)) over(order by dt) employees,
                   from_dt
             from  t
             group by dt,
                   from_dt
select  to_char(dt,'FMMonth, YYYY') month,
        employees
  from  s
  where dt >= from_dt
  order by dt
MONTH            EMPLOYEES
March, 2013              5
April, 2013              6
May, 2013                6
June, 2013               6
July, 2013               6
August, 2013             6
September, 2013          6
October, 2013            6
November, 2013           6
December, 2013           6
10 rows selected.
SQL>
SY.