所以我有一个表格，我用来绘制一些数据。
id、agcid、ooscount、日期
数据是 18 个对象每 5 分钟采样一次
ID 是自动递增的，agcid 代表对象的 ID，date 是自解释的，ooscount 是一个整数列。
我目前正在使用这个:
$sql2 = "SELECT ooscount, date
FROM ooscount
where agcid = '".$agcid."' AND date >= DATE_SUB(NOW( ),INTERVAL 7 HOUR)
order by date DESC";
简单但有效，但是我有一个非常大的数据集，使图表复杂化。
我尝试将 AND ooscount.id mod 3 = 0 添加到此，但是，每个 agcid 上的数据集似乎非常随机，有些只有一对，有些比应有的多 5 倍。
然后我尝试了这样的方法:
$sql2 = "set @row:=-1;
SELECT ooscount.*
FROM
ooscount
INNER JOIN
(
SELECT id
FROM
(
SELECT @row:=@row+1 AS rownum, id
FROM
(
SELECT id, date, ooscount FROM ooscount where agcid = '".$agcid."' and date >= DATE_SUB(NOW( ),INTERVAL 12 HOUR) order by date DESC
) AS sorted
) as ranked
WHERE rownum % 3 = 0
) AS subset
ON subset.id = ooscount.id;";
它已被注释掉，请原谅//。然而，这种方法在 phpMyAdmin 中工作时，给了我这个:
PHP Fatal error: Call to a member function fetch_assoc()
on a non-object in /var/www/khamlin/oosagc.php on line 145
最后，显示这个的代码部分:
$result2 = $conn->query($sql2);
$rowsarray = "";
//var_dump($result2);
while ($row2 = $result2->fetch_assoc()) {
我认为问题在于我发现的示例不期望单个表上有多个数据集，因此 ID 将是增量的(即:1、2、3、4、5、6 等)，而不是( 168006、168023、168041、168060)。
我将如何改变它以满足我在这里的需求？
结构
CREATE TABLE IF NOT EXISTS `ooscount` (
`id` int(10) NOT NULL AUTO_INCREMENT,
`agcid` int(3) NOT NULL,
`ooscount` int(10) NOT NULL,
`date` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 AUTO_INCREMENT=168580 ;````
样本数据:https://jpst.it/1bfFt
当前输出:每 5 分钟一个 ooscount 和日期的数据点(即:
data: [{"ooscount":"4907","date":"2018-02-21 09:40:01"},{"ooscount":"4905","date":"2018-02-21 09:35:02"},{"ooscount":"4910","date":"2018-02-21 09:30:02"},{"ooscount":"4904","date":"2018-02-21 09:25:01"},{"ooscount":"4900","date":"2018-02-21 09:20:02"},{"ooscount":"4898","date":"2018-02-21 09:15:02"},{"ooscount":"4989","date":"2018-02-21 09:10:01"},{"ooscount":"5008","date":"2018-02-21 09:05:02"},{"ooscount":"4933","date":"2018-02-21 09:00:01"},{"ooscount":"4915","date":"2018-02-21 08:55:01"},{"ooscount":"4903","date":"2018-02-21 08:50:02"},{"ooscount":"4898","date":"2018-02-21 08:45:01"},{"ooscount":"4899","date":"2018-02-21 08:40:01"},{"ooscount":"4909","date":"2018-02-21 08:35:01"},{"ooscount":"4918","date":"2018-02-21 08:30:01"},{"ooscount":"4906","date":"2018-02-21 08:25:02"},{"ooscount":"4906","date":"2018-02-21 08:20:01"},{"ooscount":"4900","date":"2018-02-21 08:15:02"},{"ooscount":"4895","date":"2018-02-21 08:10:01"},{"ooscount":"4883","date":"2018-02-21 08:05:02"},{"ooscount":"4881","date":"2018-02-21 08:00:01"},{"ooscount":"4879","date":"2018-02-21 07:55:02"},{"ooscount":"4883","date":"2018-02-21 07:50:02"},{"ooscount":"4875","date":"2018-02-21 07:45:02"},{"ooscount":"4875","date":"2018-02-21 07:40:01"},{"ooscount":"4879","date":"2018-02-21 07:35:02"},{"ooscount":"4882","date":"2018-02-21 07:30:01"},{"ooscount":"4888","date":"2018-02-21 07:25:01"},{"ooscount":"4877","date":"2018-02-21 07:20:01"},{"ooscount":"4879","date":"2018-02-21 07:15:02"},{"ooscount":"4884","date":"2018-02-21 07:10:02"},{"ooscount":"4886","date":"2018-02-21 07:05:02"},{"ooscount":"4904","date":"2018-02-21 07:00:02"},{"ooscount":"4906","date":"2018-02-21 06:55:01"},{"ooscount":"4904","date":"2018-02-21 06:53:32"},{"ooscount":"4904","date":"2018-02-21 06:50:02"},{"ooscount":"4892","date":"2018-02-21 06:45:01"},{"ooscount":"4795","date":"2018-02-21 06:40:02"},{"ooscount":"4793","date":"2018-02-21 06:35:01"},{"ooscount":"4799","date":"2018-02-21 06:30:02"},{"ooscount":"4797","date":"2018-02-21 06:25:02"},{"ooscount":"4802","date":"2018-02-21 06:20:01"},{"ooscount":"4801","date":"2018-02-21 06:15:01"},{"ooscount":"4792","date":"2018-02-21 06:10:02"},{"ooscount":"4798","date":"2018-02-21 06:05:02"},{"ooscount":"4797","date":"2018-02-21 06:00:01"},{"ooscount":"4798","date":"2018-02-21 05:55:02"},{"ooscount":"4794","date":"2018-02-21 05:50:01"},{"ooscount":"4796","date":"2018-02-21 05:45:02"},{"ooscount":"4804","date":"2018-02-21 05:40:02"},{"ooscount":"4803","date":"2018-02-21 05:35:01"},{"ooscount":"4809","date":"2018-02-21 05:30:02"},{"ooscount":"4811","date":"2018-02-21 05:25:01"},{"ooscount":"4810","date":"2018-02-21 05:20:01"},{"ooscount":"4831","date":"2018-02-21 05:15:02"},{"ooscount":"4839","date":"2018-02-21 05:10:02"},{"ooscount":"4859","date":"2018-02-21 05:05:01"},{"ooscount":"4859","date":"2018-02-21 05:00:01"},{"ooscount":"4858","date":"2018-02-21 04:55:02"},{"ooscount":"4858","date":"2018-02-21 04:50:02"},{"ooscount":"4863","date":"2018-02-21 04:45:02"},{"ooscount":"4868","date":"2018-02-21 04:40:01"},{"ooscount":"4872","date":"2018-02-21 04:35:01"},{"ooscount":"4868","date":"2018-02-21 04:30:02"},{"ooscount":"4867","date":"2018-02-21 04:25:02"},{"ooscount":"4870","date":"2018-02-21 04:20:01"},{"ooscount":"4866","date":"2018-02-21 04:15:02"},{"ooscount":"4864","date":"2018-02-21 04:10:02"},{"ooscount":"4863","date":"2018-02-21 04:05:01"},{"ooscount":"4874","date":"2018-02-21 04:00:02"},{"ooscount":"4881","date":"2018-02-21 03:55:01"},{"ooscount":"4850","date":"2018-02-21 03:50:02"},{"ooscount":"4846","date":"2018-02-21 03:45:01"},{"ooscount":"4847","date":"2018-02-21 03:40:02"},{"ooscount":"4850","date":"2018-02-21 03:35:02"},{"ooscount":"4845","date":"2018-02-21 03:30:01"},{"ooscount":"4847","date":"2018-02-21 03:25:02"},{"ooscount":"4848","date":"2018-02-21 03:20:01"},{"ooscount":"4847","date":"2018-02-21 03:15:02"},{"ooscount":"4852","date":"2018-02-21 03:10:03"},{"ooscount":"4854","date":"2018-02-21 03:05:01"},{"ooscount":"4864","date":"2018-02-21 03:00:01"},{"ooscount":"4867","date":"2018-02-21 02:55:02"},{"ooscount":"4868","date":"2018-02-21 02:50:01"},{"ooscount":"4862","date":"2018-02-21 02:45:02"}],目标是在相同的时间内拥有更少的数据点。
最佳答案
我找到了一个适合我的解决方案。
$sql2 = "SELECT *
FROM (
SELECT @row := @row +1 AS rownum, ooscount.*
FROM (
SELECT @row :=0) r, ooscount where agcid='".$agcid."' and date >= DATE_SUB(NOW( ),INTERVAL 12 HOUR)
) ranked
WHERE rownum %3 =1";
关于PHP/MySQL 第 n 行与 DATE_SUB 等，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/48912118/