Si entendí bien, creo que tu problema es exactamente el el mismo que el anterior . Solo te falta una cláusula WHERE.
A ver si esto funciona:
$con = new mysqli("localhost" ,"root" ,"" ,"user_databases");
//query buildings table for the dropdown
$bquery = mysqli_query($con, "SELECT building_ID, building_name FROM buildings");
$selectedbldg = null;
// if the form was submitted
if (!empty($_POST['bldg'])) {
// store selected building_ID
$selectedbldg = $_POST['bldg'];
// the subquery is used to count how many times each location appears
// for a particular building
$count = mysqli_query($con, "
SELECT lo.location_ID, lo.location_name, dt.num_visits
FROM location lo
JOIN (
SELECT location_ID, COUNT(location_ID) AS num_visits
FROM delivery_transaction
WHERE building_ID = {$selectedbldg}
GROUP BY location_ID
) AS dt ON lo.location_ID = dt.location_ID
");
// like before, better to use prepared statement
}
?>
<!-- ... -->
<section class="row text-center placeholders">
<div class="table-responsive">
<table class="table table-striped">
<thead>
<tr>
<th>Location</th>
<th>Number of Visits</th>
</tr>
</thead>
<tbody>
<!-- PHP alternative syntax for control structures: easier to read (imo) -->
<!-- isset function is to ensure variable $count exist as it only gets declared in the IF condition (you would get an error otherwise) -->
<!-- mysqli_num_rows is to check if there are any results to loop over -->
<?php if (isset($count) && mysqli_num_rows($count)) : ?>
<?php while($row = mysqli_fetch_assoc($count)) : ?>
<tr>
<td><?= $row['location_ID'] ?></td>
<td><?= $row['num_visits'] ?></td>
</tr>
<?php endwhile ?>
<?php else : ?>
<tr>
<td>No results to display</td>
</tr>
<?php endif ?>
</tbody>
</table>
</div>
</section>
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