Getting 'meta_value' when searching for matching 'post_id' and 'meta_key', 'meta_value' combo

postmeta table example

The end result is matching an array of 'post_id', sorted by the number of attendees. I know it is a WP backend but I have to do it in SQL, no 'get_posts' or anything WordPress related. What I'm wanting to do is confusing so I'll try to be clear.

What I Have To Begin:

• $check_post_ids - the original array of post_ids I need to check
• $start_date - the 'meta_value' each event's '_StartDate' needs to match.

What I Need To Do:

I need to check these three post_ids to see if they have a matching start date. If they do, I need to get an array of post_ids sorted from the highest to lowest number of attendees.

Currently I was planning on doing this with multiple SELECTs and foreach() statements, but I feel like there has to be a simpler way to do this ( i.e. One SELECT & foreach() ). Here's what I'm doing at the moment. I haven't finished it yet because I feel like there has to be a simpler way to do this. Newer to SQL and any help is tremendously appreciated!

    $check_post_ids = array('484', '627', '982', '2435');       
    $start_date = '1963-10-20 19:30:00';            

    // iterate through array of post_ids and check if they have the same _StartDate
    foreach($check_post_ids as $id){
        $start_date_check = "SELECT * FROM `wp_postmeta` WHERE `post_id` =" . $id . " AND `meta_key` LIKE '_StartDate' AND `meta_value` = '" . $start_date . "'";
        $start_date_check_result = mysqli_query($conn, $start_date_check);

        // assign all post_ids with a matching _StartTime to a new array
        if (mysqli_num_rows($start_date_check_result) > 0) {
            while($row = mysqli_fetch_assoc($start_date_check_result)) {
                $matching_post_ids[] = $row['post_id'];

                // iterate through matching_post_ids array, get the _NumAttendees for each, and assign their id and _NumAttendees to an assoc array to be sorted
                foreach($matching_post_ids as $id){
                    $attendees_check = "SELECT meta_value FROM wp_postmeta WHERE post_id = " . $id . " AND meta_key = '_ecp_custom_2'";
                    $attendees_check_result = mysqli_query($conn, $attendees_check);
                    if($upvotes_check > 0){
                        while($row = mysqli_fetch_assoc($attendees_check_result)){
                            $sort_by_attendees['id'] = $id;
                            $sort_by_attendees['attendees'] = $row['meta_value'];
                            $sort_by_attendees_array[] = $sort_by_attendees;
                        }
                    }
                }