Hi Haidong,

You answered my question with...

"that is normal, because some columns are not applicable to certain events".

After I posted my question I found 2 very useul articles...

http://www.sqlmag.com/Articles/Index.cfm?ArticleID=23016

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/adminsql/ad_mon_perf_6koi.asp

The second one details which columns are applicable for given events.

Thanks for your help and reply.