example 1:
$sql->setWhere(array('id' => 3, 'field' => '')); // results in id = 3 AND field = ''
$sql->setWhere(array(array('id' => 3, 'field' => ''))); // results in id = 3 OR field = ''
example 2:
$sql->setWhere('myid = :id OR anotherfield = :field', array('id' => 3, 'field' => ''));
example 3 (deprecated):
$sql->setWhere('myid="35" OR abc="zdf"');