PHP/MySql simpler more efficient code - php

Question

I am currently using three different queries to output information from a db.. I've put a sample of what I'm doing below, I'm just curious if there is a more efficient simpler way of spitting out this same info.
I have a db table for events, that has a date field, and parent event field. I have to group the events by their date, spit out the events per each date, then spit out each sub-event underneath those events (by using the parent event field).
Below is a sample of how my code it layed out, I'm just not that good in mysql if there is an easier query etc.
Thanks
$result = mysql_query("SELECT * FROM events GROUP BY date");
while ($event_date = mysql_fetch_assoc($result)) {
echo 'Date Header';
$result2 = mysql_query("SELECT * FROM events WHERE date = '" . $event_date['date'] . "' && parent_id = ''");
while ($event = mysql_fetch_assoc($result2)) {
echo 'Event display code';
$result3 = mysql_query("SELECT * FROM events WHERE date = '" . $event_date['date'] . "' && parent_id = '" . $event['id'] . "'");
while ($event = mysql_fetch_assoc($result3)) {
echo 'Sub Event code';
}
}
}
To achieve a markup like the below.
<h3>Date 1</h3>
<div class="top-level-event">
Some code
</div>
<div class="sub-level-event">
some sub level code
</div>
<div class="sub-level-event">
some sub level code
</div>
<h3>Date 2</h3>
<div class="top-level-event">
Some code
</div>
<div class="sub-level-event">
some sub level code
</div>
<div class="sub-level-event">
some sub level code
</div>

Answer 1

You can use one query:
SELECT * FROM events e1
LEFT JOIN events e2 ON e1.id=e2.parent_id AND e1.date=e2.date
WHERE e1.parent_id=''
ORDER BY e1.date