In qlikview also you can use quite similar syntax.
In load script you can write
load a, b
where not isnull(a);
the same not isnull(a) can be used in charts also with if condition,
suppose you want sum of b when a is not null then,
Also, as others said sometimes (when loading data from text files) isnull() may not work then you can use len() function. The reason for isnull() not working is that by definition text files can not contain null data.