PostgreSQL中按组分组的月份之间的月份差异

Question

我有使用PostgreSQL的数据，看起来像这样

customer name       order_id         order_date
John                A001             1-Jan-2017
John                A002             1-Feb-2017
John                A003             1-Apr-2017
Smith               A004             1-Dec-2016
Smith               A005             1-Feb-2017
Jane                A006             1-Mar-2017
Dave                A007             1-Feb-2017
Dave                A008             1-Feb-2017
Dave                A009             1-Feb-2017

我试图在另一列中获得回购月份之间的差额。 像这样

customer name       order_id         order_date        month_diff
John                A001             1-Jan-2017        null
John                A002             1-Feb-2017        1
John                A003             1-Apr-2017        2
Smith               A004             1-Dec-2016        null
Smith               A005             1-Feb-2017        3
Jane                A006             1-Mar-2017        null
Dave                A007             1-Feb-2017        null
Dave                A008             1-Feb-2017        0
Dave                A009             1-Feb-2017        0

任何建议将不胜感激。 我是PostgreSQL的新手。 先感谢您

Answer 1

这可以使用窗口函数轻松完成（假设order_date正确定义为DATE ）

select customer_name,
       order_id, 
       order_date, 
       order_date - lag(order_date) over (partition by customer_name order by order_date) as diff
from order_table
order by customer_name, order_date;

请注意，如果order_date是date ，则差异结果以天为单位。

Answer 2

尝试 first_value() ：

select 
customer_name, order_id, order_date
, order_date - first_value(order_date) over (partition by customer_name order by order_id) as month_diff
from tname;

Answer 3

尝试这个：

select
t.customer,
t.order_id,
t.order_date,
(select extract(MONTH from t.order_date) - extract(MONTH from tt.order_date) 
    from your_table tt
    where tt.order_id < t.order_id
    order by tt.order_id desc limit 1
)
from your_table t
order by t.order_id