Creating these binary flags, as FHoeglund has mentioned is an excellent way of doing exactly what you're looking for. The only thing I would add/change from what was already said is how to refer to it within your expressions.
FHoeglund wrote:Then refer to the flags in your expression. E.g. sum(if(FLAG_MTD_LastYear='1',Sales)
Since your flags are binary you can multiply them by the field you want to sum and the only rows that get summed are the ones for which the binary flag equals '1'. (ie. Sum(FLAG_MTD_LastYear * Sales) ). This method is quite a bit faster than IF() statements because of the difference in operations required by the CPU.
John Witherspoon has also recently posted experimental results on performance differences between using IF, multiplication and set analysis using binary flags (Set analysis example: Sum( {$<FLAG_MTD_LastYear={1}>} Sales) ). His post is here:
http://community.qlik.com/forums/p/17188/71371.aspx
His findings seem to indicate Set analysis is faster than the other two methods. I have not been able to verify this, but I can say that switching from If statements to multiplication of binary flags gave us HUGE performance benefits.
Hopefully this helps,
sjprows
