Why is ISNULL functionality not working?

[Keerthi]

I am trying to get the data only when my lastlogon(field name) is Null. but the above query is still giving me data for both Null and non Null values.

[woodcock]

Fields are case sensitive and also sometimes "empty" (i.e. == "").  You can check for both like this:

(isnull(LASTLOGON) OR LASTLOGON=="")

[ITWhisperer]

Field name are case-sensitive - try this

| where isnull(LASTLOGON)