比较当前月份的订单日数与上次飞蛾的每日计数

Question

我想比较本月每日订单数量与上个月每日订单数量之间的差异，并使用postgresql sql查询按天分组。

示例：

1st of nov count - 1st of oct count

Answer 1

只需计算上个月的日期并加入

create table sales (
   d DATE
  ,orederCount INT 
);

INSERT INTO sales(d, orederCount) VALUES  ('2016-11-01',12),('2016-10-01',10),('2016-09-01',10)
                                         ,('2016-11-02',13),('2016-10-02',14),('2016-09-02',5);   



with data as (select d,(d-INTERVAL '1 month')::DATE as p,orederCount from sales)
select current.d,current.orederCount - COALESCE(prev.orederCount,0) as diff 
from data as current 
LEFT JOIN data as prev ON current.p = prev.d
;

结果：

2016-09-01,10
2016-09-02,5
2016-10-01,0
2016-10-02,9
2016-11-01,2
2016-11-02,-1