You'd do something like:
select * from SMS_R_System where SMS_R_System.ResourceId NOT IN ( SELECT SMS_G_System_SERVICE.ResourceId FROM SMS_G_System_SERVICE WHERE SMS_G_System_SERVICE.DisplayName = "SMS Agent Host" )
However, as you rightly point out, no SCCM client = no hardware inventory = no service information.
What you're better off doing is using information on discovered devices in your site with no client:
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.SMSAssignedSites = "XXX" and SMS_R_System.Client = 0
Replacing "XXX" with your site code.
Hope this helps.