PHP从关联表的列中查找数字的平均值

Question

我有可能误解了这个问题。

我有两个表，用户和评论。 Stars是Reviews表中的一列。

我只是希望能够显示用户的平均星级评分。我无法弄清楚如何访问这些然后找到平均值。

这是我到目前为止所建立的：

        // Get Data
    $qry  = "SELECT * FROM `users` LEFT JOIN `reviews` ON users.userId = reviews.user_id WHERE installation_id = $installation_id";
    $res = mysqli_query($mysqli, $qry) or die('-1'.mysqli_error($mysqli));

    //$uqry = "SELECT membership FROM users WHERE userId = $uid";
    //$current_user = mysqli_query($mysqli, $uqry) or die('-1'.mysqli_error($mysqli));

    $getUser = mysqli_fetch_assoc(mysqli_query($mysqli, "SELECT membership FROM users WHERE userId = $uid"));
    $currentUserLevel = $getUser['membership'];
$user_review_id = 60;


?>
<div class="container">
    <div class="content">
        <?php if ($msgBox) { echo $msgBox; } ?>
        <div class="row">       

        <?php $lists = array(); 
            while($list = mysqli_fetch_assoc($res)) {
                  $lists[] = $list;
                }

                foreach($lists as $list) { 
                        $name = stripslashes($list['usersName']); 
                        $bio  = stripslashes($list['usersBio']); 

 ?>


                    <div class="col-md-4">
                        <div id = "user-square">  
                                <div class="avatar">
                                    <img src="<?php echo $avatarDir.$list['usersAvatar']; ?>" class="publicAvatar" />
                                </div>
                                Name:<?php echo $name; ?> <br /> 
                                Bio:<?php echo $bio;  ?> <br />      
                                Stars: <?php echo $stars;  ?> <br />                    

                                <?php 
                                    if ($currentUserLevel == 'pro') {
                                    echo 'CONTACT SCOUT';
                                }
                                    else {
                                    echo 'Sorry you must upgrade to a Pro membership to contact this Scout';    
                                }   
                                ?>
                        </div>      
                    </div>
            <?php   }
                ?>
            </div>
        </div>      

    </div>
</div>