Question

我有一个包含多个时间戳条目的表，我正在尝试列出与每个唯一bookID条目的最新更新时间戳相关联的数据。

该表格如下：

create table rankHistory (
id int(10),
rankUpdated timestamp,
bookId varchar(10),
currentRank int(5)
);

示例数据：

INSERT INTO rankHistory
(`id`, `rankUpdated`, `bookId`,`currentRank`)
VALUES
(1, '2016-05-27 05:11:44', 'BK-001','100'),
(2, '2016-05-28 08:11:44', 'BK-001','95'),
(3, '2016-05-29 07:11:44', 'BK-001','11'),
(4, '2016-05-29 08:11:44', 'BK-001','5'),
(5, '2016-05-27 05:11:44', 'BK-002','100'),
(6, '2016-05-27 05:11:44', 'BK-002','100'),
(7, '2016-05-29 05:11:44', 'BK-002','100'),
(8, '2016-05-29 06:11:44', 'BK-002','55')
;

合并以下sqlFiddle中的示例： http://sqlfiddle.com/#!9/4b02d3/4

我正在尝试输出如下：

2016-05-29 08:11:44, BK-001, 5
2016-05-29 06:11:44, BK-002, 55

我试过并且认为可以使用的查询（我对mySQL不是很熟悉所以请温柔......）

select bookId,currentRank,MAX(rankUpdated) FROM rankHistory group by bookId

这确实显示了最新的时间戳条目，但没有显示匹配的数据。实现上述输出的正确方法是什么？

Answer 1

您可以使用NOT EXISTS()选择仅记录的记录，其中包含较新时间戳的其他记录不存在。

SELECT * FROM rankHistory t
WHERE NOT EXISTS(SELECT 1 FROM rankHistory s
                 WHERE s.bookId = t.bookId AND
                       s.rankUpdated > t.rankUpdated)

Answer 2

尝试以下查询：

select Max(RankUpdated),A.bookId,A.CurrentRank from (
select bookId,currentRank,RankUpdated FROM rankHistory 
where RankUpdated IN (select MAX(rankUpdated) FROM rankHistory group by CurrentRank)
group by bookId,currentRank) A
group by A.bookId

它将提供如下所需的输出： -

 RankUpdated          bookId    currentRank 
 2016-05-29 08:11:44  BK-001        5
 2016-05-29 06:11:44  BK-002        55

Answer 3

这将有效：

select r.bookId, r.currentRank, r.rankUpdated from rankHistory r
inner join (
    select id, bookId, MAX(currentRank) from (
        select id, bookId, currentRank from rankHistory
        order by rankUpdated desc
    )  t group by t.bookId
) t on t.id = r.id

Answer 4

使用join：

select a.bookId,a.currentRank,a.rankUpdated from rankHistory a join (
select bookId,MAX(rankUpdated) as maxrank FROM rankHistory group by bookId) b on a.bookID = b.bookId and a.rankUpdated=b.maxrank

看小提琴：http://sqlfiddle.com/#!9/4b02d3/9

mySQL如何正确排序这些数据？

4 个答案: