查询时，auto_increment默认值为NULL的列为NULL

I have the a column in my table called spon_applicant_ID, it looks like this:

The AUTO_INCREMENT default value is working just fine:

But when I try to pull the value from the database, it is NULL:

$conn = new PDO('mysql:host=localhost;dbname=dbname;charset=utf8', 'user', 'PW');
$conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

$sql = 'SELECT * FROM spon_applicants sa LEFT OUTER JOIN spon_results sr ON sa.spon_applicant_ID = sr.spon_applicant_ID WHERE sr.spon_results_ID IS NULL ';
$stmt = $conn->prepare($sql);
$stmt->execute();

if($stmt->rowCount() > 0){
    $results = $stmt->fetchAll(PDO::FETCH_ASSOC);
}

var_dump($results);

My results look like:

array(17) {
  [0]=>
  array(37) {
    ["spon_applicant_ID"]=>
    NULL
    ["application_date"]=>
    string(19) "2016-02-04 16:41:16"

And so on.

How can I use reference the spon_applicant_ID field from the database? Is my understanding of AUTO_INCREMENT correct, as it is the mySQL equivalent as an identity?

From your last comments:

@devpro, could you make this an answer? This was correct. Thank you. – etm124 1 hour ago

You need to use alias along with column name because your column names are same in both tables.

Example:

SELECT sa.spon_applicant_ID ... 

Use only LEFT JOIN instead of LEFT OUTER give null when doesn't have the equivalent value