"IS NULL" in Zend_Db_Table select not working

I'm trying to do a join on 2 tables in Zend, using the DbTable 开发者_如何学JAVA/ model / mapper structure. If, in my mapper, I do this:

$select = $this->getDbTable()->select(Zend_Db_Table::SELECT_WITH_FROM_PART)
        ->setIntegrityCheck(false)
        ->join('images', 'images.oldFilename =
                                               availablePictures.filename')
               ->where('images.ref IS NOT NULL');
$resultSet = $this->getDbTable()->fetchAll( $select );

it works like a charm, but if I try the same thing with IS NULL instead of NOT NULL, I get nothing where I should get a result set of several rows, just like when I try it directly in MySQL with

SELECT *
FROM (
`availablePictures` AS a
LEFT JOIN `images` AS i ON a.filename = i.oldFilename
)
WHERE i.`ref` IS NULL

I get the impression Zend doesn't like my IS NULL or am I doing something wrong?

The solution was to be found in Machine's comment on my original post. Doing what he suggested I noticed that Zend created an inner join as I was using the wrong select method, so:

$select = $this->getDbTable()->select(Zend_Db_Table::SELECT_WITH_FROM_PART)
        ->setIntegrityCheck(false)
        ->joinLeft('images', 'images.oldFilename =
                                               availablePictures.filename')
               ->where('images.ref IS NOT NULL');
$resultSet = $this->getDbTable()->fetchAll( $select );

is how it should be.

My thinking is it has to do with the way MySql decides what is NULL and what isn't. Is it possible that the results you are expecting have a default assignment of the empty string '' or 0 in the images.ref column? MySql does not treat those as NULLs. Have a look here:

http://dev.mysql.com/doc/refman/4.1/en/working-with-null.html