Do you mean hide the entire column if every value in the column has a value of NULL? Not to be showed when there's any value in there would be always hide the column; so rather than hiding it just don't put it in the report.
You can hide a by selecting it in your tablix/matrix, right clicking ans selecting properties. Then go to Visibility and you can enter an expression there. Note that TRUE = hide and FALSE = visible.
Excuse my typos and sometimes awful grammar. My fingers work faster than my brain does :-P
Please always remember to encapsulate your code in IFCode Markup. For example [code=sql] [/code]
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