if condition in sql server update query

Question

I have a SQL server table in which there are 2 columns that I want to update either of their values according to a flag sent to the stored procedure along with the new value, so

Answer 1

The current answers are fine and should work ok, but what's wrong with the more simple, more obvious, and more maintainable:

IF @flag = 1
    UPDATE table_name SET column_A = column_A + @new_value WHERE ID = @ID;
ELSE
    UPDATE table_name SET column_B = column_B + @new_value WHERE ID = @ID;

This is much easier to read albeit this is a very simple query.

Here's a working example courtesy of @snyder: SqlFiddle.