agrupando arquivo por ano e mês usando php e mysql

$data = array();
while($row = $stmt->fetch()){
  $monthName = date("F", mktime(0, 0, 0, $row['Month'], 10));     
  $data[$row['Year']][$monthName][] = array(
   'post' => $row['postTitle'],
   'slug' => 'a-'.$row['Month'].'-'.$row['Year']
  );
}
echo '<ul>';
foreach ($data as $year => $yearData){
   echo "<li>$year<br/>";
   echo '<ul>';
   foreach ($yearData as $month => $monthData){
      echo "<li>$month<br/>";
      echo '<ul>';
      foreach ($monthData as $number => $postData){
        echo "<li><a href='${postData['slug']}'>Post $number</a><br/>";
        echo "<a href='#'>${postData['post']}</a></li>";
      }
      echo '</ul></li>';
   }
   echo '</ul></li>';
}
echo '</ul>';

SELECT
    Year(postDate) as Year, Month(postDate) as Month,
    GROUP_CONCAT(postTitle) as posts
FROM
    blog_posts_seo
GROUP BY Year, Month
ORDER BY postDate DESC

• GROUP BY e GROUP_CONCAT