Here is the code in order to use Sum and case to group results.
SELECT SUM (CASE when sales_date >= '2003-01-01' and
sales_date <= '2003-01-31' then qty else 0 end)
SUM (CASE when sales_date >= '2003-02-01' and
sales_date <= '2003-02-28' then qty else 0 end)
Just to avoid loosing lost hair (from pulling it out), I'd tweak your syntax just a smidgeon.
SELECT SUM (CASE when sales_date >= '2003-01-01'
and sales_date < '2003-02-01' then qty end) as 'Jan'
, SUM (CASE when sales_date >= '2003-02-01'
and sales_date <= '2003-03-01' then qty end) as 'Feb'
This gets you around needing to figure out how many days are in which months (which is just lazy on my part), but it also prevents you from loosing any data entered with a time on the last day of the month too!