General sql query question (compare date ranges)

Dave_be

I'm trying to create a report true SQL BI that compares the sales of a product/customer between two given date frames.
For example, I want to see the difference in sale from item "111" from (01/01/2008 till 31/12/2008) and (01/01/2009 until 31/01/09.)
All data is in the same table, so no external joins are needed.

Data output must give something like this:

Item     Date 1       Date 2      Difference
111       2000           1000           -100% 

But the second thing is, when date 1 has no data for the period it still must show date 2.

Item   Date 1      Date 2      Difference
111       0            1000          100% 

I just can't seem to get this in my head and this is really frustrating me for quite some time now.
Any help would be highly appreciated

bbrown

Use a left outer join between the two parts of your query. This way it returns an item only if it has data for the first range.