Get Meta Value for all Posts via $wpdb

I wrote a function which should give you the value of a meta key for all of the posts. I am not sure why my query is not working. It gives me an empty array as output when I var_dump the query. The value of the meta key is stored in array, so I am using unserialize to convert the mysql array to PHP array.

The Function

function wp_postquiz_total_completed_quizes_by_user( ) {

    global $wpdb;

    $mylink = $wpdb->get_results("
                        SELECT $wpdb->postmeta.meta_value * 
                        FROM $wpdb->postmeta 
                        WHERE $wpdb->postmeta.meta_key = '_pq_users_answered_quiz_on_post'",

    $array = unserialize($mylink);

    return $array;

Source: wpdb

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