mysql - 计算 SQL 中日期之间的平均时间

Question

使用 MySQL，我试图弄清楚如何回答这个问题:创建第 N 个项目的用户之间的平均月数是多少？
预期结果:

| project count | Average # months |
| 1             | 0                | # On average, it took 0 months to create the first project (nothing to compare to)
| 2             | 12               | # On average, it takes a user 12 months to create their second project
| 3             | 3                | # On average, it takes a user 3 months to create their third project

我的 MySQL 表代表用户创建的项目。该表可以概括为:

| user_id | project created at |
|---------|--------------------|
| 1       | Jan 1, 2020 1:00 pm|
| 1       | Feb 2, 2020 3:45 am|
| 1       | Nov 6, 2020 0:01 am|
| 1       | Mar 4, 2021 5:01 pm|
|------------------------------|
| 2       | Another timestamp  |
| 2       | Another timestamp  |
| 2       | Another timestamp  |
| 2       | Another timestamp  |
| 2       | Another timestamp  |
| 2       | Another timestamp  |
|------------------------------|
| ...     | Another timestamp  |
| ...     | Another timestamp  |

有些用户只有一个项目，而有些用户可能有数百个。
编辑:当前实现

with
    paid_self_serve_projects_presentation as (
        select 
                `Paid Projects`.owner_email
            `Owner Email`, 
                row_number() over (partition by `Paid Projects`.owner_uuid order by created_at)
            `Project Count`,
                day(`Paid Projects`.created_at)
            `Created Day`,
                month(`Paid Projects`.created_at)
            `Created Month`,
                year(`Paid Projects`.created_at)
            `Created Year`,
                `Paid Projects`.created_at
            `Created`
        from self_service_paid_projects as `Paid Projects`
        order by `Paid Projects`.owner_uuid, `Paid Projects`.created_at
    )
    
select `Projects`.* from paid_self_serve_projects_presentation as `Projects`

Answer 1

您可以使用窗口函数。我在想 row_number()枚举按创建日期排序的每个用户的项目，以及lag()获取上一个项目的创建日期:

select rn, avg(datediff(created_at, lag_created_at)) avg_diff_days
from (
    select t.*,
        row_number() over(partition by user_id order by created_at) rn,
        lag(created_at, 1, created_at) over(partition by user_id order by created_at) lag_created_at
    from mytable t
) t
group by rn

这为您提供了平均天数差异，这在某种程度上更准确。如果你真的想要几个月，那么使用 timestampdiff(month, lag_created_at, created_at)而不是 datediff() - 但请注意，该函数返回一个整数值，因此会损失精度。