Get latest entry based on date

问题

I want to get the info about users and the amount of points they had before a specific date. The problem I can't know when their state was last recorded so I have to get the latest entry before the date.

The query I've ended up with is something like this (simplified example):

SELECT u.id, p.points, p.timestamp
FROM user AS u 
  INNER JOIN points 
    ON u.id = p.user_id
WHERE u.group = 'red'
AND p.timestamp::date <= '2018-01-01 10:00:00'
GROUP BY u.id, u.first_name, u.last_name, mh.gamified_mastery, mh.updated_at
ORDER BY mh.updated_at DESC

This gives me a table like this:

id | points | timestamp
-----------------------
 1 | 10     | 2018-01-01 9:00:00
 1 | 25     | 2018-01-01 8:57:00
 1 | 25     | 2018-01-01 8:00:00
 2 | 100    | 2018-01-01 7:00:00
 2 | 50     | 2018-01-01 6:00:00
 2 | 15     | 2018-01-01 5:55:00

I don't actually want the timestamp to be displayed, it's here for presentation's sake. I only want the top entries for each player but I have to group by timestamp for the ORDER BY to work.

Due to other limitations I really need to get all of this done in one query (I know I could just do a separate query with LIMIT 1 and join them in the app but that's not an option currently).

回答1:

I think you want distinct on:

SELECT DISTINCT ON (u.id) u.id, . . .
FROM (SELECT u.id, p.points, p.timestamp
      FROM user u INNER JOIN
           points p
           ON u.id = p.user_id
      WHERE u.group = 'red' AND
            p.timestamp::date <= '2018-01-01 10:00:00'
      GROUP BY u.id, u.first_name, u.last_name, mh.gamified_mastery, mh.updated_at
    ) pu
ORDER BY u.id, mh.updated_at DESC;

来源：https://stackoverflow.com/questions/49812971/get-latest-entry-based-on-date

标签

sql

postgresql

greatest-n-per-group