SWQL any formal linkage between Orion.Nodes and IPAM.IPNode?

I'm attempting to get a list of IPAM entries that need some attention and/or cleanup using the following:

SELECT DISTINCT

  I.IPAddress

, I.DnsBackward

, O.Caption

, O.IP_Address

FROM IPAM.IPNode AS I

LEFT OUTER JOIN Orion.Nodes AS O ON I.IPAddress = O.IP_Address

WHERE I.Status = 1

AND ( I.DnsBackward LIKE '' OR I.DnsBackward IS NULL)

What I see is lots of mismatches when I attempt to link using IP Address.  So several questions:

1. Is there any formal linkage (key) between these two tables?

2. If there is no formal linkage, Is there a better one? 

3. Is there some place I can see linkages between Orion and IPAM tables?

Thanks in advance.