Filter list on field not displayed in list

[geertdeman]

In qlik sense I want to filter a list in a report based on a column (Filtercol) that is not in the list (or should not be visible in the list).  I used another column (showfield) to display the vaule of showfield column  if the condition is not true, otherwise Null value will be returned.  In the list I exclude the null values for this column

The following condition is working but is very slow:  if(Filtercol='UNKNOWN',null(),showfield)

[dplr-rn]

Why not use set analysis something like
Filtercol-={'UNKNOWN'}

[Vegar]

Try

only({<Filtercol-={'UNKNOWN'}>}showfield)