如何使用PHP中的一个mySQL数据库表处理两组大型数据？

Question

I'm attempting to work with two sets of data from the same mySQL table in a PHP script. The idea is data is scraped from an API and into a database hourly. A second script then pulls the information out of the database and displays a rolling 6-hour delta.

I've run into a bit of a problem trying to create the delta from the two datasets. I need to run two mySQL queries to get the data I need (current and from 6 hours ago), but can't really think of a way to get the script to work without including the queries inside the loops that output each entry (These can run up to a couple of hundred times, and I don't think having that many mySQL queries running would be good?)

This is what I have so far:

//select the system table and pulls data acquired within the last hour.
$sql = "SELECT system, vp, vpthreshold, owner, time FROM SysData WHERE time > DATE_SUB(NOW(),INTERVAL 1 HOUR)";
$result = $conn->query($sql);

if ($result->num_rows > 0) {
    // output data of each row
    while($row = $result->fetch_assoc()) {

    //Calculates contested percentage
    $vpthreshold = $row["vpthreshold"]; 
    $vp = $row["vp"];   
    $currentcontested = $vp/$vpthreshold*100;

    //Catches potential divide by zeroes, echos system is stable.
    if ($vp == 0.0){
        echo $row["system"] . " " . "is Stable<br>";
    }
    //else output contested percentage with system name in readable format.
    else{

        echo $row["system"] . " " . "{$currentcontested}%" . "<br>";
    }
} 
}

There's a broadly identical statement that pulls and echos the second set of information underneath this. How can I get these two sets together so I can work with them? Very new to PHP and learning on the fly here.

Answer 1

You can look into nested queries. Something like the following:

SELECT (data_now.somevalue - data_prev.somevalue) as deltavalue FROM
(
    (your first select statement) as data_now, 
    (your 6 hours ago select statement) as data_prev
);

This lets you select data from other select statements all in one go. The 2 inner "select statements" you should replace with your respective queries. The results will be put temporarily into data_now and data_prev. You can then use these as normal tables in the outer select statement.

EDIT: To be more specific to what you want, here is an updated example:

SELECT (data_now.vp/data_now.vpthreshold - data_prev.vp/data_prev.vpthreshold) as deltavalue FROM
(
    (SELECT system, vp, vpthreshold, owner, time FROM SysData WHERE time > DATE_SUB(NOW(),INTERVAL 1 HOUR)) as data_now, 
    (your 6 hours ago select statement) as data_prev
);

In your PHP code remember to reference the result as:

$row["deltavalue"]

or whatever you put after "as" in the outer SELECT.