Oracle date function for the previous month

Question

I have the query below where the date is hard-coded. My objective is to remove the harcoded date; the query should pull the data for the previous month when it runs.

select count(distinct switch_id)   from [email protected]  where dealer_name =  'XXXX'    and TRUNC(CREATION_DATE) BETWEEN '01-AUG-2012' AND '31-AUG-2012' 

Should I use sysdate-15 function for that?

Answer 1

Modifying Ben's query little bit,

 select count(distinct switch_id)      from [email protected]    where dealer_name =  'XXXX'        and creation_date between add_months(trunc(sysdate,'mm'),-1) and last_day(add_months(trunc(sysdate,'mm'),-1)) 

Answer 2

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 [email protected]    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().

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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