如何选择此循环中的所有其他结果？[PHP]

Question

我有一个输出到图的循环。随着时间的推移，我的“历史”表中的数据急剧增长，我希望通过选择其他的结果来减少输出到图表上的数量。

我怎样才能用我的代码做到这一点呢？

<?php

    $cdate = 0;   
    $counter = 0;

    $sql = "SELECT * FROM history WHERE time < '$ctime' ORDER BY time asc";
    $result = $conn->query($sql);

    if ($result->num_rows > 0) {
        while($row = $result->fetch_assoc()) {
            $cdate = ($row["time"]);

            $sql1 = "SELECT AVG(value) AS avgval FROM history WHERE time = '$cdate' AND time < '$ctime'";
            $result1 = $conn->query($sql1);

            if ($result1->num_rows > 0) {  
                while($row1 = $result1->fetch_assoc()) {   
                    $avgval = $row1["avgval"];
                    echo "[new Date((".($row["time"] * 1000).")), ".$avgval."],";    

                }  
            }
        }     
    }
?>

例如，如果我的表(历史记录)中有这样的内容：

id -时间

1- Group1 - 100 -- 730

2-第2组- 200 - 730

3-第1组- 110 - 740

第二组-- 235 -- 740

5-第1组- 310 - 750

6-2组- 485 - 750

我如何才能输出1，2和5，6的平均值--忽略#4和#5？

Answer 1

<?php
$cdate = 0;   
$counter = 0;

$sql = "SELECT * FROM history WHERE time < '$ctime' ORDER BY time asc";
$result = $conn->query($sql);

if ($result->num_rows > 0) {
    while($row = $result->fetch_assoc()) {
        $cdate = ($row["time"]);

        $sql1 = "SELECT AVG(value) AS avgval FROM history WHERE time = '$cdate' AND time < '$ctime'";
        $result1 = $conn->query($sql1);

        $i = 0; // incrementor
        if ($result1->num_rows > 0) {  
            while($row1 = $result1->fetch_assoc()) {   
                ++$i; // plus one
                if($i % 2 == 0) { // check if $i is even          
                    $avgval = $row1["avgval"];
                    echo "[new Date((".($row["time"] * 1000).")), ".$avgval."],";    
                }
            }  
        }
    }     
}
?>