根据两个字段获取最新记录

Question

我有一个包含以下字段的 table：

user_id         year      month     amount    type
--------------------------------------------------
5               2018      1         100       Foo
6               2017      12        100       Bar
6               2018      11        100       Foo
6               2018      12        100       Bar
7               2018      12        100       Foo
7               2019      12        100       Bar
8               2019      12        100       Foo

我想做 3 件事：

获取某些用户的每个用户id的最新记录（1条记录，最新年份和月份字段），

类似于：

select *
from myTable
where user_id in (6,7) and <is latest year / month>

哪个应该return

user_id         year      month     amount     type
---------------------------------------------------
6               2018      12        100        Bar
7               2019      12        100        Foo

计算上述查询的总数，类似于：

    select SUM(amount) as myTotal, avg(amount) as myAverage, 
    (count # of foos in result) as numberOfFoos,
    (count # of bars in result) as numberOfBars
    from myTable
    where user_id in (6,7) and <is latest year / month>

这将 return 包含自定义字段的一行：

myTotal     myAverage     numberOfFoos      numberOfBars
--------------------------------------------------------
300         100           2                 1

与上述查询相同，但针对一段时间内的每个月，例如。在过去的 3-5 年中，最好是尽可能少的调用，而不是手动循环 36 个月以上并进行单独调用。

    year     month    myTotal     myAverage     numberOfFoos      numberOfBars
    --------------------------------------------------------------------------
    2018     1        300         100           2                 1
    2018     2        300         100           2                 1
    2018     3        300         100           2                 1
    ...
    2020     12       300         100           2                 1

Answer 1

第一个问题可以使用 row_number 函数解决：

with d as (
  select 
    users.*, 
    row_number() over (partition by user_id  order by year desc, month desc ) rn 
  from  users 
  where  user_id in (5, 6)
) 
select * from d
where  rn = 1;

PostgreSQL fiddle here

第二个问题可以用同样的方法解决：

with d as (
  select 
    users.*, 
    row_number() over (partition by user_id  order by year desc, month desc) rn 
  from  users 
  where  user_id in (6, 7)
) 
select 
    sum(amount), avg(amount), 
    count(*) filter (where type = 'Foo') count_foo, 
    count(*) filter (where type = 'Bar') count_bar
from d
where  rn = 1;

Test solution here

Answer 2

select year, month, sum(amount), avg(amount),
       sum(case when type = 'Foo' then 1 else 0 end) as num_foos,
       sum(case when type = 'Bar' then 1 else 0 end) as num_bars
from (select u.*, 
             row_number() over (partition by user_id  order by year desc, month desc ) as seqnum 
      from users 
      where user_id in (5, 6)
     ) u
where seqnum = 1
group by year, month;

根据两个字段获取最新记录

Get latest record based on two fields

sql

postgresql

greatest-n-per-group