how to filter empty string

[Dan Yi]

Hi,

I have data as following :

Timestamp  itemid  actiontype

"2012-11-01T23:59:59.000+0000"  ""      "Search"
"2012-11-01T23:59:59.000+0000"  "688531"        "ShowItemDetail"
..

I want to filter out all the lines that have itemid as empty,
I tried different approach but none of work:

C = FILTER B BY itemid != '';
C = FILTER B BY itemid != null;


Anyone knows how to do it? Thanks.