Načtení každého 30. záznamu z databáze

SET @counter=0;
SELECT id, value, @counter rownum FROM price HAVING (@counter:=@counter+1)%30=0 ORDER BY id DESC LIMIT 100

<?php
$url = 'www.xxx.com';
$obj = json_decode(file_get_contents($url), true);
 
//db
$conn = new mysqli("localhost", "root", "", "aa");
 
// Stochastic K index
$Stochastic_k_15m = array();

$intervals = [1, 15, 60, 1440];

$min = array_fill(0, count($intervals), PHP_INT_MAX);
$max = array_fill(0, count($intervals), PHP_INT_MIN);
 
// výběr posledních 3000 položek z databaze, ale kazde 30 polozky podle ID a casu
$result = $conn->query("SELECT value FROM price ORDER BY id DESC LIMIT 20160");

for($i = 0; $row = $result->fetch_assoc(); $i++) {
    foreach($intervals as $index => $interval ) {
        if($i%$interval === 0) {
            if($row['value'] < $min[$index]) {
                $min[$index] = $row['value'];
            }
            if($row['value'] > $max[$index]) {
                $max[$index] = $row['value'];
            }
        }
    }
}

foreach($intervals as $index => $interval ) {
    echo "Stochastic výsledek pro interval $interval minut: " . 100*(($obj['lastPrice']-$min[$index])/($max[$index]-$min[$index])) . "<br>\n";
}
?>

SELECT t.*,
       p2.value first_value
FROM   (SELECT intervaly.sekund,
               intervaly.popis,
               p.date,
               Min(p.time)  min_time,
               Min(p.value) min_value,
               Max(p.value) max_value,
               Avg(p.value) avg_value
        FROM   price p
               CROSS JOIN (SELECT 'minuty' popis,
                                  60       sekund
                           FROM   DUAL
                           UNION ALL
                           SELECT 'ctvrthodiny' popis,
                                  15 * 60       sekund
                           FROM   DUAL
                           UNION ALL
                           SELECT 'hodiny' popis,
                                  60 * 60  sekund
                           FROM   DUAL
                           UNION ALL
                           SELECT 'dny'        popis,
                                  24 * 60 * 60 sekund
                           FROM   DUAL) intervaly
        WHERE  p.date BETWEEN Subdate('2018-05-08', 13) AND Date('2018-05-08')
        GROUP  BY intervaly.popis,
                  intervaly.sekund,
                  p.date,
                  Time_to_sec(p.time) DIV intervaly.sekund) t
       JOIN price p2
         ON t.date = p2.date
            AND t.min_time = p2.time
ORDER  BY t.sekund DESC,
          t.date,
          t.min_time  

SELECT MIN(value), MAX(value)
FROM (SELECT value FROM price WHERE date > NOW() - INTERVAL 14 DAY ORDER BY date DESC) t;

SELECT MIN(value), MAX(value)
FROM (SELECT value FROM price WHERE date > NOW() - INTERVAL 14 DAY AND MINUTE(date) in (0, 15, 30, 45) ORDER BY date DESC) t;

SELECT MIN(value), MAX(value)
FROM (SELECT value FROM price WHERE date > NOW() - INTERVAL 14 DAY AND MINUTE(date) = 0 ORDER BY date DESC) t;

SELECT MIN(value), MAX(value)
FROM (SELECT value FROM price WHERE date > NOW() - INTERVAL 14 DAY AND HOUR(date) = 0 AND MINUTE(date) = 0 ORDER BY date DESC) t;

SELECT min(`value`),
       max(`value`)
FROM   `price`
WHERE  `date` > Now() - INTERVAL 14 day
       AND Minute(`date`) IN ( 0, 15, 30, 45 )

<?php
 
// připojení k databázi
$conn = new mysqli("localhost", "root", "", "");
 
$url = 'https://www.xxx.com/';
$obj = json_decode(file_get_contents($url), true);
 
// Výpočet Stochastic - K indexu
// Vzorec: %K = 100((C-Lx)/(Hx-Lx))
// Kde: Lx je nejnižší bod (low) za vybraný časový úsek, Hx je nejvyšší bod (high) za vybraný časový úsek, C je poslední zavírací cena (close) na příslušném časovém období (timeframu),
function stochastic_formula($C, $Lx, $Hx, $scale=1) {
    return round(100*(($C-$Lx)/($Hx-$Lx)), $scale);
}

function stochastic_from_db($conn, $C, $add_condition='', $days=14) {
    $data = $conn->query("SELECT MIN(value) minv, MAX(value) maxv FROM price WHERE date > Now() - INTERVAL {$days} day {$add_condition}")->fetch_assoc();
    return stochastic_formula($C, $data['minv'], $data['maxv']);
}

//Stochastic 1 minuta - 14 dnů
$Stochastic_K_1minuta_vypocet = stochastic_z_db($conn, $obj['lastPrice']);
//Stochastic 15 minut - 14 dnů
$Stochastic_K_15minut_vypocet = stochastic_z_db($conn, $obj['lastPrice'], "AND Minute(date) IN (0,15,30,45)");
//Stochastic 1 hodina - 14 dnů
$Stochastic_K_1hodina_vypocet = stochastic_z_db($conn, $obj['lastPrice'], "AND MINUTE(date) = 0");
//Stochastic 1 den - 14 dnů
$Stochastic_K_1den_vypocet = stochastic_z_db($conn, $obj['lastPrice'], "AND HOUR(date) = 0 AND MINUTE(date) = 0");