strange database problem to debug

I am developing a dynamic web page using Drupal 7. I ran into a very strange problem. I have reduced my problem to a very small test case as follows:

The database side: I am using MySQL. I have two tables as defined here with a few sample data entries:

CREATE TABLE people (
  pid INT(6) UNSIGNED AUTO_INCREMENT PRIMARY KEY,
  name VARCHAR(30) NOT NULL
);
INSERT INTO people VALUES (NULL, 'Joe');
INSERT INTO people VALUES (NULL, 'Ant');
INSERT INTO people VALUES (NULL, 'Tom');

CREATE TABLE event (
  eid INT(6) UNSIGNED AUTO_INCREMENT PRIMARY KEY,
  pid INT(6) UNSIGNED NOT NULL,
  event_desc VARCHAR(20) NOT NULL,
  event_date DATE NOT NULL
);
INSERT INTO event VALUES (NULL, 1, '1p', '2015-10-01');
INSERT INTO event VALUES (NULL, 2, '1p', '2015-10-12');
INSERT INTO event VALUES (NULL, 2, '2p', '2015-10-00');
INSERT INTO event VALUES (NULL, 2, '3p', '2015-00-00');
INSERT INTO event VALUES (NULL, 3, '1p', '2010-07-18');
INSERT INTO event VALUES (NULL, 3, '2p', '2010-09-00');

Note the only interesting feature here is that the event table may contain incomplete event date in the format of '2015-10-00' when date is unknown and '2015-00-00' when both date and month are unknown. I understand this is legal in MySQL.

The Drupal 7 side: I create a custom module as follows:

<?php
/**
 * Implements hook_menu().
 */
function test_menu() {
  $items['test'] = array(
    'type' => MENU_NORMAL_ITEM,
    'title' => 'Test',
    'description' => 'Test',
    'page callback' => '_test_page',
    'access callback' => TRUE,
    'menu_name' => 'main-menu',
  );
  return $items;
}

function _test_page() {
  $output = "";
  $sql = "SELECT event.event_desc, event.event_date, people.name FROM  event, ";
  $sql .= "people WHERE event.pid=people.pid ORDER BY people.name";
  $result = db_query($sql);
  foreach ($result as $data) {
    $output .= $data->name. " ". $data->event_desc. " ". $data->event_date. "<br>\n";
  }
  return $output;
}

Note that I create a page with a menu item 'Test'. The page performs a query and displays the result. Now the very strange part: when the event date is incomplete, the query returns the wrong data '0000-00-00'. I am expecting this result:

Ant 2p 2015-10-00
Ant 3p 2015-00-00
Ant 1p 2015-10-12
Joe 1p 2015-10-01
Tom 1p 2010-07-18
Tom 2p 2010-09-00

I am getting this INCORRECT result:

Ant 1p 2015-10-12
Ant 2p 0000-00-00
Ant 3p 0000-00-00
Joe 1p 2015-10-01
Tom 2p 0000-00-00
Tom 1p 2010-07-18

I tries to isolate the problem. Here are my observations:
- My SQL statement appears to be correct. I ran it direct from mysql command line to get the correct result.
- I wrote a standalone PHP code and pass my query directly to PDO and I again got the correct result.
- The error only occurs when I do a join in my query. If accessing the 'event' table only I can get those correct dates returned.
- Given the above, is there a problem in Drupal 7's database abstraction layer?