Mysql组由一个字段和另一个字段组成

Question

也许这个问题是重复的，但我找不到我的任务的答案。我有下表的数据：

我需要选择具有唯一oid的数据，并为每个oid选择最大日期。我有这些查询，但它们的工作不正确：

SELECT `oid`, `pvalue`, `date`
FROM `report`
WHERE `deviceid` = 'MRA-1011'
  AND `date` <= '2012-01-20 00:00:00'
GROUP BY oid
HAVING `date` = MAX(`date`)
ORDER BY `date` DESC;

SELECT `oid`, `pvalue`
FROM `report`
WHERE `deviceid` = 'MRA-1011'
  AND `date` <= '2012-01-20 00:00:00'
GROUP BY oid
ORDER BY `date` DESC;

我想得到这个结果：

Answer 1

您需要使用以下行的子选择：

SELECT oid, pvalue FROM report
JOIN (SELECT oid, MAX(date) AS maxDate FROM report r WHERE `deviceid` = 'MRA-1011'
AND `date` <= '2012-01-20 00:00:00' GROUP BY oid) AS foo
ON foo.oid=r.oid AND foo.maxDate = report.date;

如果report.date不是唯一的，您可能希望在外部查询

中对其进行分组

Answer 2

试试这个::

SELECT `oid`, `pvalue`, `date`, MAX(`date`)
FROM `report`
WHERE `deviceid` = 'MRA-1011'
AND `date` <= '2012-01-20 00:00:00'
GROUP BY oid
HAVING `date` = MAX(`date`)
ORDER BY `date` DESC;

Answer 3

SELECT `oid`, `pvalue`
FROM `report` AS `r1`
WHERE `deviceid` = 'MRA-1011'
  AND `date` <= '2012-01-20 00:00:00'
  AND `date` = ( SELECT MAX(`date`) 
                 FROM `report` AS `r2` 
                 WHERE `r1`.`oid` = `r2`.`oid`
               )
;

Answer 4

为了得到这个结果，你必须

ORDER BY `date` ASC:

SELECT `oid`, `pvalue`
FROM `report`
WHERE `deviceid` = 'MRA-1011'
AND `date` <= '2012-01-20 00:00:00'
GROUP BY oid
HAVING `date` = MAX(`date`)
ORDER BY `date` ASC;