计算月流失率

Question

我正在尝试计算每月流失率（给定月份：number_of_people_who_unsubscribed / number_of_subscribers_at_beginning_of_month）。

我有一个 subscribers 表，如下所示：

id	start_date	end_date
1	2020-03-17	null
2	2020-06-21	2020-09-03

我可以使用这样的查询计算一个月的流失率：

select
(
  /* Subscriptions that ended during January */
  select count(*)::decimal from subscriptions
  where end_date is not null
  and end_date >= '2021-01-01'
  and end_date <= '2021-01-31'
) /
(
  /* Subscriptions that were active at the beginning of January */
  select count(*)::decimal from subscriptions
  where end_date is null
  or end_date >= '2021-01-01'
) as churn

这给了我一月份退订用户的单一百分比。但是，我想每个月输出这个百分比，所以我可以将它显示为折线图。我不知道从哪里开始——感觉就像我需要循环并运行每个月的查询，但感觉不对。我如何进行相同的计算，但不手动指定月份？我们可以假设每个月至少有一个 start_date 和一个 end_date，所以某种group by 可能会起作用。

最终，我正在寻找类似于以下内容的输出：

month	churn
2020-03	0.076
2020-04	0.081
2020-05	0.062

Answer 1

使用您的数据逻辑和月份序列：

select to_char(running_month, 'yyyy-mm') as "month",
(
  /* Subscriptions that ended during January */
  select count(*)::numeric from subscriptions
  where end_date is not null
  and end_date >= running_month
  and end_date <= running_month + interval '1 month - 1 day'
) /
(
  /* Subscriptions that were active at the beginning of January */
  select count(*)::numeric from listings
  where end_date is null
  or end_date >= running_month
) as churn
from generate_series ('2020-01-01'::date, '2020-12-01'::date, interval '1 month') as running_month;