mysql pedigree - unknown ancestors

[Tanja]

I´m using Barands pedigree to draw, but if there are unknown ancestors it won´t be work correct.

 

// print parent trees, sire then dam
		 if ($N < $max) {
			 $sql = "SELECT a.father_id, a.mother_id, s.dogname, d.dogname
					 FROM dog a
					    INNER JOIN dog s ON a.father_id = s.id
					    INNER JOIN dog d ON a.mother_id = d.id
					 WHERE a.id = '$dogid' ";
			 $res = mysql_query($sql);
			 list($s, $d, $sn, $dn) = mysql_fetch_row($res);
			 printTree($s, $sn, $N+1, $max);
			 printTree($d, $dn, $N+1, $max);
		 }

For example, if a dog has no father and the father_id is NULL -> possible mother isn´t show too (and calculation of avk isn´t right (is 200% should be 100%)).
If I change the unknown father_id to 0 it will be the same.
If i change to an id which stands for unknown father (for exampe 500) it will be all correct - but if there are more than one ancestors missing, calculating is bad ....

My questions: How is the best way to handle unknown ancestors? For each dog seperatly unknown father / mother? Or other way?
And how to change code above?