How to write query for multisite use

I’m trying to create a plugin for use on our multisite install, but I can’t quite figure out how to the db query in a way that will automatically get data from the correct subsite’s db tables.

Here’s a query that works:

$reportList = $wpdb->get_results($wpdb->prepare("SELECT wp_2_watupro_taken_exams.ID,
wp_2_watupro_taken_exams.exam_id,
wp_2_watupro_taken_exams.user_id,
wp_2_watupro_taken_exams.end_time 'completed',
wp_2_watupro_master.name 'assessment',
wp_users.display_name 'name',
wp_users.user_email 'email'
FROM wp_2_watupro_taken_exams
JOIN wp_2_watupro_master ON wp_2_watupro_master.ID = wp_2_watupro_taken_exams.exam_id
JOIN wp_users ON wp_users.ID = wp_2_watupro_taken_exams.user_id
WHERE wp_users.user_email LIKE '%".$_POST['client']."%'
ORDER BY wp_users.display_name;"));

I want to make it so that I don’t have the “wp_2_” in there — i.e., so that it will just automatically get the results based on the right prefix for the current site.

I’ve done a bunch of searching and reading, and here’s what I’ve got (which doesn’t work):

$reportList = $wpdb->get_results($wpdb->prepare("SELECT $wpdb->watupro_taken_exams.ID,
$wpdb->watupro_taken_exams.exam_id,
$wpdb->watupro_taken_exams.user_id,
$wpdb->watupro_taken_exams.end_time 'completed',
$wpdb->watupro_master.name 'assessment',
$wpdb->users.display_name 'name',
$wpdb->users.user_email 'email'
FROM $wpdb->watupro_taken_exams
JOIN $wpdb->watupro_master ON $wpdb->watupro_master.ID = $wpdb->watupro_taken_exams.exam_id
JOIN $wpdb->users ON $wpdb->users.ID = $wpdb->watupro_taken_exams.user_id
WHERE $wpdb->users.user_email LIKE '%".$_POST['client']."%'
ORDER BY $wpdb->users.display_name;"));

So, the question is – where have I gone wrong?