选择时间戳单元格上的最新条目

Question

我有这两个表：

User:
=======================
  id  |  Name | Email 
=======================
  1   | User-A| a@mail
  2   | User-B| b@mail
=======================

Entry:
=================================================
  id  |  agree |       createdOn        | userId
=================================================
  1   | true   | 2020-11-10 19:22:23    |   1
  2   | false  | 2020-11-10 22:22:23    |   1
  3   | true   | 2020-11-11 12:22:23    |   1
  4   | true   | 2020-11-04 22:22:23    |   2
  5   | false  | 2020-11-12 02:22:23    |   2
================================================

我需要得到以下结果：


=============================================================
  Name    |   Email    |   agree     |       createdOn           
=============================================================
 User-A   |  a@mail    |   true      |  2020-11-11 22:22:23
 User-B   |  b@mail    |   false     |  2020-11-12 02:22:23
=============================================================

我运行的 Postgres 查询是：

select distinct on (e."createdOn", u.id)
u.id , e.id ,u."Name" , u.email, e.agree, e."createdOn" from "user" u
inner join public.entry e on u."id" = e."userId"
order by "createdOn" desc

但问题是它returns所有条目在做join之后！我只想要 createdOn 单元格的最新条目。

Answer 1

您想要每个用户的最新条目。为此，您需要 distinct on 子句中的用户 ID，而不是其他列。这保证每个用户在结果集中一行。

然后，您需要将该列放在 order by 子句的最前面，然后是 createdOn desc。这打破了联系并决定哪一行将保留在每组中：

select distinct on (u.id) u.id , e.id ,u."Name" , u.email, e.agree, e."createdOn" 
from "user" u
inner join public.entry e on u."id" = e."userId"
order by u.id, "createdOn" desc

Answer 2

您还可以使用 row_number 到 select 最新行，然后进行连接

SELECT * FROM USER A 
LEFT JOIN (
  SELECT * FROM (
    SELECT *,ROW_NUMBER() OVER(PARTITION BY USERID ORDER BY CREATEDON DESC) AS RN
      FROM ENTRY
  ) K WHERE RN = 1
) B
ON A.ID = B.USERID

Answer 3

尝试使用 createdon = max(createdon) 函数。按用户分组

选择时间戳单元格上的最新条目

Selecting the most recent entry on a timestamp cell

sql

postgresql

datetime

inner-join

greatest-n-per-group