Как лучше сделать выборку из двух таблиц?

<?php
        $records = array();
        $sel = $mysqli->query('SELECT * FROM Предмет');
        while( $row = $sel->fetch_assoc() ){
            $records[] = $row;
        }
        foreach ($records as $row): ?>
    <b><?=$row['Предмет']?></b>
    <ul>
        <?php 
        $id_item = $row['id'];
        $loc = array();
        $sel2 = $mysqli->query("SELECT * FROM Наименование WHERE ID_Предмета = ".$id_item);
        while( $row2 = $sel2->fetch_assoc() ){
            $loc[] = $row2;
        }
        foreach ($loc as $row2): ?>
        <li><?=$row2['Наименование']?></li>
        
   
        <?php endforeach ?> 
    </ul>
    <?php endforeach ?>

$begin = microtime(1);
echo 'BEGIN: ' . $begin . '<br>';
$query = $conn->query('select c.name as category, c.id as category_id, a.title as article from categories as c
    left join articles as a on c.id = a.category_id ');
 
$articles = array();
while ($row = $query->fetch_assoc()) {
    //echo $row['category_id']. '<br>';
    if (!empty($row['article']))$articles[$row['category_id']][] = $row['article'];
    else $articles[$row['category_id']] = array();
}
 
 
$cCategories = 0;
$cArticles = 0;
foreach ($articles as $category => $categoryArticles) {
    //echo $category . '--><br>' ;
    $cCategories++;
    foreach ($categoryArticles as $article) {
        //echo '&nbsp&nbsp&nbsp&nbsp' . $article . '<br>';
        $cArticles++;
    }
}
$end = microtime(1);
echo 'END:' . $end . '<br>';
$result = $end - $begin;
echo 'END - BEGIN: '. $result .'<br>';
echo '<b> MY RESULT WITH ' . $cCategories . ' CATEGORIES AND ' .$cArticles. ' ARTICLES =
<span style="color:red;">' . $result .'</span></b><br><br><br>';
 
/***
 *
 *   MULTIPLY QUERIES
 */
 
$begin2 = microtime(1);
echo 'BEGIN: ' . $begin2 . '<br>';
$query2 = $conn->query('select name , id from categories');
$stmt = $conn->prepare('SELECT title as name from articles where category_id = ?');
 
$cCategories = 0;
$cArticles = 0;
while ($rowC = $query2->fetch_assoc()) {
    //echo $rowC['name']. '--><br>' ;
    $cCategories ++;
    $stmt->bind_param('i', $rowC['id']);
    $stmt->execute();
    $res = $stmt->get_result();
    while ($rowA = $res->fetch_assoc()) {
        //echo '&nbsp&nbsp&nbsp&nbsp' . $rowA['name'] . '<br>';
        $cArticles++;
    }
}
 
$end2 = microtime(1);
echo 'END2:' . $end2 . '<br>';
$result2 = $end2 - $begin2;
echo 'END2 - BEGIN2: '. $result2 .'<br>';
echo '<b> YOUR RESULT WITH ' . $cCategories . ' CATEGORIES AND ' .$cArticles. ' ARTICLES =
<span style="color:red;">' . $result2 .'</span></b>';