如何按日期顺序计算日期之间的平均天数

Question

Table contains order dates

create table orderdate ( orderdate date not null )

How to calculate average days between dates in table. Difference needs to be calculated between neighbour dates in date order to find average number of dates between orders.

Adding this number to last order date produces estimated next order date.

For example if table contains

2019-09-10
2019-09-21
2019-09-25

differences between orders are 10 and 4 days and average difference is

(10+4)/2 = 7 days

next order esimated date is 2019-09-25 + 7 = 2019-10-02

Using

PostgreSQL 9.4.19

Answer 1

You can use aggregation:

select ( max(date) - min(date) ) / nullif(count(*) - 1, 0) as avg_duration
from t;

The number of gaps is one less than the count. Hence the average is the total duration divided by the number of gaps.

Answer 2

First calculate the difference between adjacent dates, then take the average:

SELECT avg(CAST(days_between AS double precision))
FROM (SELECT orderdate
             - (lag(orderdate) OVER (ORDER BY orderdate)) AS days_between
      FROM orderdate) AS q;

I cast to double precision , because the difference between two date s is an integer .