PHP assign result from nested query into array

Instead of doing several MySQL queries in a loop, consider constructing such a query, which will return all the results you need.

SQL

First of all, it would make sense to use the GROUP SQL construct.

SELECT
    s.Date       date,
    MIN(s.Speed) min,
    MAX(s.Speed) max,
    AVG(s.Speed) avg
FROM speed_table s
WHERE s.Date IN (SELECT DISTINCT d.Date FROM date_table d)
GROUP BY s.Date

It is important to understand what each part of this query does. If you run into any problems, consult the MySQL reference manual.

PHP

Forget about using mysql_* functions: they are deprecated and better implementations have emerged: PDO and mysqli. My example uses PDO.

try {
    $dbh = new \PDO('mysql:dbname=my_db_name;host=127.0.0.1', 'dbuser', 'dbpass');
} catch (\PDOException $e) {
    echo 'Connection failed: ' . $e->getMessage();
}

$sql  = '...';  // This is the query
$stmt = $pdo->prepare($sql);
$stmt->execute();

You can now either iterate over the returned result set

while ($row = $stmt->fetch()) {
    printf("%s; %s; %s; %s\n", $row['date'], $row['min'], $row['max'], $row['avg']);
}

or have the Statement object return the whole array by using

$result = $stmt->fetchAll(\PDO::FETCH_ASSOC);

I would first of all save yourself a nested query by doing the first query as:

$result = mysql_query("select Date, min(Speed) as Min_spd, max (Speed) as Max_spd, avg(Speed) as Avg_spd from ... group by Date")

And then create a running total for the sums as you're looping through each row:

$sumMinSpeeds=0;
$sumMaxSpeeds=0;
$sumAvgSpeeds=0;
while($row = mysql_fetch_array($result))
{
    echo row['Min_spd'];
    echo row['Max_spd'];
    echo row['Avg_spd'];

    $sumMinSpeeds += $row['Min_spd'];
    $sumMinSpeeds += $row['Max_spd'];
    $sumMinSpeeds += $row['Avg_spd'];

}
echo $sumMinSpeeds;
echo $sumMaxSpeeds;
echo $sumAvgSpeeds;

I don't know if the MySQL SUM function can be used to do that on te query, try it. But on this example, using arrays you should store the values and then use array_sum to return the sum of the elements. Something like this:

$min_spd = array();
$max_spd = array();
$avg_spd = array();
while($row = mysql_fetch_assoc($result))
{
    $min_spd[] = $row['Min_spd'];
    $max_spd[] = $row['Max_spd'];
    $avg_spd[] = $row['Avg_spd'];
}

echo array_sum($min_spd);

It can also be done using variables to store and add the values:

$min_spd = 0;
$max_spd = 0;
$avg_spd = 0;

while($row = mysql_fetch_assoc($result))
{
    $min_spd += $row['Min_spd'];
    $max_spd += $row['Max_spd'];
    $avg_spd += $row['Avg_spd'];
}

echo $min_spd;

$data = array();
while ($row = mysql_fetch_assoc($result)) {
    $data[] = $row;
}

then later on you can do

foreach($data as $row) {
   echo $row['Min_spd'];
   echo ...
   ...
}