Message-ID: <0dd4a9d2-1b88-cb9b-c84a-6124d658a82b@joeconway.com>
Date: Wed, 8 Jun 2022 10:15:58 -0400
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101
 Thunderbird/91.9.1
Content-Language: en-US
To: "David G. Johnston" <david.g.johnston@gmail.com>, stercor@gmail.com
Cc: pgsql-sql <pgsql-sql@lists.postgresql.org>
References: 
 <CAPDmd-CjfhW5pSkthOGmb1rDpznDWYOetrhVh1gcquwQM9n5eA@mail.gmail.com>
 <CAKFQuwaY-zeoWkJwqDP6TDZnowdif2AZsT28+-SA2OWc8KXKTA@mail.gmail.com>
From: Joe Conway <mail@joeconway.com>
Subject: Re: Electricity bill
In-Reply-To: 
 <CAKFQuwaY-zeoWkJwqDP6TDZnowdif2AZsT28+-SA2OWc8KXKTA@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit
Archived-At: 
 <https://www.postgresql.org/message-id/0dd4a9d2-1b88-cb9b-c84a-6124d658a82b%40joeconway.com>
Precedence: bulk

On 6/7/22 21:47, David G. Johnston wrote:
> On Tue, Jun 7, 2022 at 6:39 PM Theodore M Rolle, Jr. <stercor@gmail.com 
> <mailto:stercor@gmail.com>> wrote:
> 
>     I'm putting in YYYY-MM-DD dates of electricity bills and would like
>     to not have starting and ending dates in the same row. Only ending date.
>     Will someone show me the SQL to compute the months’ usage? This
>     requires retrieving two rows to compute the number of days...
> 
> You can use a window function called lead (or lag) to retrieve a value 
> from the next (previous) row and associate it with the current row.

Another way that might work for you is interval math and ranges, e.g.:

select enddt, month, year from bill;
              enddt             | month | year
-------------------------------+-------+------
  2021-12-31 23:59:59.999999-05 | dec   | 2021
  2022-01-31 23:59:59.999999-05 | jan   | 2022
  2022-02-28 23:59:59.999999-05 | feb   | 2022
  2022-03-31 23:59:59.999999-04 | mar   | 2022
  2022-04-30 23:59:59.999999-04 | apr   | 2022
  2022-05-31 23:59:59.999999-04 | may   | 2022
  2022-06-30 23:59:59.999999-04 | jun   | 2022
  2022-07-31 23:59:59.999999-04 | jul   | 2022
  2022-08-31 23:59:59.999999-04 | aug   | 2022
  2022-09-30 23:59:59.999999-04 | sep   | 2022
  2022-10-31 23:59:59.999999-04 | oct   | 2022
  2022-11-30 23:59:59.999999-05 | nov   | 2022
  2022-12-31 23:59:59.999999-05 | dec   | 2022
(13 rows)

WITH tsr (tr, month, year) AS
(
  SELECT
   tstzrange(b.enddt - '1 month'::interval,
             b.enddt,
             '(]') AS tr,
   b.month,
   b.year
  FROM
   bill b
)
SELECT tr, month, year
FROM tsr
WHERE now() <@ tr;
-[ RECORD 1 ]----------------------------------------------
tr    | ["2022-05-30 00:00:00-04","2022-06-30 00:00:00-04")
month | jun
year  | 2022

Adjust the open/closed bounds to suit.

HTH,

-- 
Joe Conway
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com