Re: How can I get first day date of the previous month ? - Mailing list pgsql-general

On 06/20/2014 12:11 AM, Arup Rakshit wrote:

Thanks for your answer. How to get the first day date of last 6 months from now then will be :

yelloday_development=# select date_trunc('month', now()) - interval '5 month' as first_month;

        first_month        

---------------------------

 2014-01-01 00:00:00+05:30

(1 row)

Is it correct ? I am new pgdql DB :-) Awesome DB it is... 

 

Welcome. And yes, it is awesome. Being new to the DB and mailing list, please note that the convention on all PostgreSQL mailing lists is to post your reply at the bottom and not to top-post.

The solution you gave will work but I'll offer a word of caution - date and time manipulation can get tricky and even the way it is handled in PostgreSQL has occasionally been tweaked between versions (a good reason to always read the release notes).

The three things that seem to cause the most confusion are time-zones, daylight saving time and irregular intervals. So if you assume that one day is 24 hours you can encounter trouble at DST changes. And PostgreSQL, like any system that manipulates time, needs to make certain assumptions about what an interval means (what is one month before March 31) which can lead to this:

steve=> select '2014-03-31'::date - '1 month'::interval + '1 month'::interval;
---------------------
 2014-03-28 00:00:00

when you might have expected this:

steve=> select '2014-03-31'::date - ('1 month'::interval + '1 month'::interval);
---------------------
 2014-01-31 00:00:00

Have fun but read the docs, experiment and test - especially with dates and times.

Cheers,
Steve

Some SQL I wrote to explore this.

I think my solution does not have the above problems, but may have others! :-)


Cheers,
Gavin


DROP TABLE IF EXISTS datex;


CREATE TABLE datex
(
    id          int     PRIMARY KEY,
    a_date      date    NOT NULL
);


INSERT INTO datex
(
    id,
    a_date
)
VALUES
    (101, '2014-01-01'),
    (102, '2014-01-02'),
    (128, '2014-01-28'),
    (129, '2014-01-29'),
    (130, '2014-01-30'),
    (131, '2014-01-31');


SELECT
    (date_part('year', d.a_date)::text
    || '-'
    || (date_part('month', d.a_date) + 1)::text
    || '-1')::date
FROM
    datex d;