Assistance with SWQL Query for Node UP/SNMP DOWN

There is a great thread that pointed out the lovely way that one can monitor where SNMP is failing by utilizing the max DateTime from the CPULoad metrics - this is far nicer than expending monitors for SNMP verification as I have previously done (checking for SNMP over SNMP).

NPM Alert on Node UP (Ping) SNMP DOWN (unknown / unresponsive)

While the SQL queries are excellent for reporting or for generating email alerts (and seem to be working fine on my systems) - what I would really love is to add a Custom resource to my dashboard to display the servers that are currently outside of the parameters.  I'm struggling a bit with the SWQL though - I can get parts of it to work just fine, but I can't seem to bring it together into a whole.  Can anyone assist with where I've gone off track?

SELECT 
a.NodeID, 
a.Caption, 
a. Status, 
a.MachineType, 
MAX (c.DateTime) as Last_Poll 
FROM Orion.Nodes a 

JOIN Orion.CPULoad(nolock=true) c ON a.NodeID=c.NodeID 

where a.Status = 1 
and a.UnManaged = 0 
and MinuteDIFF((MAX (c.DateTime)), GetDate()) > 30