Find Pod entries Where Relationship Field Has Been Linked

I have a dogs Custom Post Type pod. In that pod are three (relevant) bi-directional relationship fields to a litters Custom Post Type pod:

• breedings-as-sire
• breedings-as-dam
• from-litter

So now I’m making pages listing dogs with this or that, and would like a page each showing all dams, sires, and puppies.

I’m stuck on creating the where clause for the pod params array. example:

$dogParams = array( 'limit' => $limitPerPage
	, 'orderby' => $aOrderBy
	, 'where' => "breedings-as-dam [what-to-put-here-to-limit-to-dogs-that-are-dams-in-1-or-more-litters?]");
$dogs = pods( 'dogs', $dogParams );

I’ve tried

, 'where' => "breedings-as-dam IS NOT NULL" ==> gives me every dog back

, 'where' => "ISNULL(breedings-as-dam,'') <> ''" ==> gives me error text which is missing the ‘<>’ portion

, 'where' => "ISNULL(breedings-as-dam,'') != ''" ==> gives me error text say8ing the ‘!=’ is not valid

I suspect I might be barking {hehhehheh} up the wrong tree here. Could you point me in the right direction?