If you are using User Device Affinity–associating a user with one or more specified devices for application deployment to make sure the application install on all devices that are associated with that user,it is necessary to identify the computers which do not have primary User Defined.More about User Define Affinity http://technet.microsoft.com/en-us/library/gg699365.aspx.
Create collection using below WQL query for list of computers with Primary Device is NULL.(Make sure you limit the collection to computers with Client installed and Active) else you will get all computers who do not have sccm client installed .
WQL query uses wmi class (SMS_UserMachineRelationship) which store the information only about UDA relationship. Usermachinerelationship will not have any entries for computers do not have UDA relationship.
so I use subselected query using not in function.Get list of computers for which UDA relationship is not null and compare them with v_r_system active clients.
select SMS_R_SYSTEM.ResourceID,SMS_R_SYSTEM.ResourceType,SMS_R_SYSTEM.Name,SMS_R_SYSTEM.SMSUniqueIdentifier,SMS_R_SYSTEM.ResourceDomainORWorkgroup,SMS_R_SYSTEM.Client from SMS_R_System where SMS_R_System.ResourceId not in (select resourceid from SMS_UserMachineRelationship where UniqueUserName is not null)
Hope it helps.
SCCM Configmgr 2012 Collection for computers with Primary User (UDA) is NULL is a post from: Eswar Koneti's Blog