How to include only null values in Set Expression

[#1]

I have a set expression that works fine. It excludes the Null values and shows the correct result

only({<ServerName-={''}>} ServerName)

I am looking for the Set Expression to return only Null values. I am not sure what am i missing here but this should work but it is not.

only({<ServerName={''}>} ServerName)

Also how do i combine multiple criteria here like "Only rows where ServerName is Null and HostName is Null"

[sasiparupudi1]

Try

only({<ServerName={"$(=Null())"}>} ServerName) 

[tresB]

May be like:

only({$ - <ServerName={'*'}, HostName={'*'}>} ServerName)

[Kushal_Chawda]

only({ <ServerName-={'*'}, HostName-={'*'}>} ServerName)

[#5]

They work individually but not when combined

[tresB · Accepted solution]

Try(not tried myself though):

only({$ - <ServerName={'*'}> * $-< HostName={'*'}>} ServerName)

Or,

only({($ - <ServerName={'*'}>) * ($-< HostName={'*'}>)} ServerName)