计算PostgreSQL数据库中的平均日期

Question

I have a table in PostgreSQL which has orders purchased at various dates. In order to archive the data we need to merge all the records of the year and merge it into one record with a average purchase date calculated based on the purchase dates.

How do I calculate the average purchase date in PostgreSQL? Or Is there a way to do it in JAVA?

Answer 1

Since your dates are all in the same year, you could use extract to convert the dates to a day-of-year, average those numbers, and then add that back to the first day of the year to get a date:

select '2015-01-01'::date
     + avg(extract(doy from whatever_your_date_column_is))::int
     - 1
from ...

You need the ::int cast because avg gives you a double precision value but there is no operator for adding doubles to dates; the -1 adjustment is needed because the day-of-year values are one-based rather than zero-based. Note that adding an integer to a date simply adds that many days.

You would, of course, adjust the '2015-01-01' to match whatever year you're working with.

Answer 2

You should convert your dates to an epoch (seconds since 1970-01-01 00:00:00), take the average of that, then turn the result back into a date:

SELECT extract(year from purchase_date) AS year_of_purchase,
       to_timestamp(avg(extract(epoch from purchase_date)))::date AS avg_purchase_date
FROM orders
GROUP BY 1;

Add other fields and filters as required.

Answer 3

java.time

In java.time , if you want the average date within a timeframe of the calendar year you could use the ZonedDateTime class and it's getDayOfYear method.

For a timeframe beyond the calendar year, you could choose a point in the past. I suggest the Java epoch of first moment of 1970 in UTC. See the constant ZoneOffset.UTC . Search StackOverflow to learn how to get the number of days from that early pint to each of your date-time instances. Average those numbers of days to arrive at your average date.