MySQL 查询，MAX() + GROUP BY

2021-11-20 00:00:00 sql database mysql relational-database

愚蠢的 SQL 问题.我有一个像这样的表('pid'是自动递增的主列)

Daft SQL question. I have a table like so ('pid' is auto-increment primary col)

CREATE TABLE theTable (
    `pid` INT UNSIGNED PRIMARY KEY AUTO_INCREMENT,
    `timestamp` TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
    `cost` INT UNSIGNED NOT NULL,
    `rid` INT NOT NULL,
) Engine=InnoDB;

实际表格数据:

INSERT INTO theTable (`pid`, `timestamp`, `cost`, `rid`)
VALUES
  (1, '2011-04-14 01:05:07', 1122, 1),
  (2, '2011-04-14 00:05:07', 2233, 1),
  (3, '2011-04-14 01:05:41', 4455, 2),
  (4, '2011-04-14 01:01:11', 5566, 2),
  (5, '2011-04-14 01:06:06', 345, 1),
  (6, '2011-04-13 22:06:06', 543, 2),
  (7, '2011-04-14 01:14:14', 5435, 3),
  (8, '2011-04-14 01:10:13', 6767, 3)
;

我想获取每个 RID 的最新行的 PID(每个唯一 RID 有 1 个结果).对于示例数据，我想:

I want to get the PID of the latest row for each rid (1 result per unique RID). For the sample data, I'd like:

pid | MAX(timestamp)      | rid
-----------------------------------
5   | 2011-04-14 01:06:06 | 1
3   | 2011-04-14 01:05:41 | 2
7   | 2011-04-14 01:14:14 | 3

我尝试运行以下查询:

SELECT MAX(timestamp),rid,pid FROM theTable GROUP BY rid

我得到:

max(timestamp)     ; rid; pid
----------------------------
2011-04-14 01:06:06; 1  ; 1
2011-04-14 01:05:41; 2  ; 3
2011-04-14 01:14:14; 3  ; 7

返回的 PID 始终是 RID 第一次出现的 PID(行/pid 1 是第一次使用rid 1，行/pid 3 第一次使用 RID 2，行/pid 7 是第一次使用rid 3用来).虽然为每个rid 返回最大时间戳，但pid 不是原始表中时间戳的pid.什么查询会给我我正在寻找的结果?

The PID returned is always the first occurence of PID for an RID (row / pid 1 is frst time rid 1 is used, row / pid 3 the first time RID 2 is used, row / pid 7 is first time rid 3 is used). Though returning the max timestamp for each rid, the pids are not the pids for the timestamps from the original table. What query would give me the results I'm looking for?

推荐答案

(Tested in PostgreSQL 9.something)

(Tested in PostgreSQL 9.something)

确定rid和时间戳.

select rid, max(timestamp) as ts
from test
group by rid;

1   2011-04-14 18:46:00
2   2011-04-14 14:59:00

加入吧.

select test.pid, test.cost, test.timestamp, test.rid
from test
inner join 
    (select rid, max(timestamp) as ts
    from test
    group by rid) maxt
on (test.rid = maxt.rid and test.timestamp = maxt.ts)

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