Oracle date function for the previous month

Modifying Ben's query little bit,

 select count(distinct switch_id)     from xx_new.xx_cti_call_details@appsread.prd.com   where dealer_name =  'XXXX'       and creation_date between add_months(trunc(sysdate,'mm'),-1) and last_day(add_months(trunc(sysdate,'mm'),-1))

The trunc() function truncates a date to the specified time period; so trunc(sysdate,'mm') would return the beginning of the current month. You can then use the add_months() function to get the beginning of the previous month, something like this:

select count(distinct switch_id)     from xx_new.xx_cti_call_details@appsread.prd.com   where dealer_name =  'XXXX'       and creation_date >= add_months(trunc(sysdate,'mm'),-1)    and creation_date < trunc(sysdate, 'mm')

As a little side not you're not explicitly converting to a date in your original query. Always do this, either using a date literal, e.g. DATE 2012-08-31, or the to_date() function, for example to_date('2012-08-31','YYYY-MM-DD'). If you don't then you are bound to get this wrong at some point.

You would not use sysdate - 15 as this would provide the date 15 days before the current date, which does not seem to be what you are after. It would also include a time component as you are not using trunc().

Just as a little demonstration of what trunc(<date>,'mm') does:

select sysdate     , case when trunc(sysdate,'mm') > to_date('20120901 00:00:00','yyyymmdd hh24:mi:ss')             then 1 end as gt     , case when trunc(sysdate,'mm') < to_date('20120901 00:00:00','yyyymmdd hh24:mi:ss')             then 1 end as lt     , case when trunc(sysdate,'mm') = to_date('20120901 00:00:00','yyyymmdd hh24:mi:ss')             then 1 end as eq  from dual       ;SYSDATE                   GT         LT         EQ----------------- ---------- ---------- ----------20120911 19:58:51                                1

Data for last month-

select count(distinct switch_id)  from xx_new.xx_cti_call_details@appsread.prd.com where dealer_name =  'XXXX'   and to_char(CREATION_DATE,'MMYYYY') = to_char(add_months(trunc(sysdate),-1),'MMYYYY');