Help query

Help query

Hi, i need help , how I could do this.

for example, I have this table

  1. Fecha      Plano      Perdida     etc     etc   etc etc etc
    01-03-2014 20 10
    02-03-2014 40 60
    03-03-2014 50 40
    04-03-2014 10 25
    05-03-2014 15 32
    06-03-2014 25 18
    07-03-2014 86 22
    08-03-2014 12 45
    09-03-2014 14 60
    10-03-2014 25 21
    11-03-2014 34 44
    12-03-2014 22 24
    13-03-2014 12 18

I need to create a new table containing the following

Fecha, Proporcion 
where fecha between 01-03-2014 and  07-03-2014 proporcion = Plano , when fecha > 07-03-2014 proporcion = Plano + Perdida

Any help will be very grateful.