I'm willing to bet it's the "distinct" item you have in there. I'm also curious
A) why you think you need it (the nodes table only holds unique records, no need to specify distinct unless you are doing some other kind of complex query)
...and...
B) how you got it in there in the first place. The top of the query - the initial "Select Nodes.NodeID as NetObjectID, Nodes.Caption as NodeName from Nodes" part is hard-coded into the alert. Where YOU are able to add stuff appears lower down where your JOIN and WHERE clauses would come in.