Query calculations when some records don't exist

[doma23]

Would it be possible later to do query calculations between two records even when one of the two records doesn't exist?

Example:

COMPANY____PERIOD___SALES

Company1___1H2012___125
Company1___1H2011___100
*Company1___1H2010____? (*this record doesn't exist at all)

Company2___1H2012___240
Company2___1H2011___220
Company2___1H2010___200


So I want to compare these records, to see the percentage increase in Sales over three periods, for each company.

I want to get something like this, period vs period comparison:

COMPANY____1H2011___1H2012
Company1___ n/a ____ 25%
Company2___ 10% ____ 20%

[Rabbit]

You would do an aggregate query and pivot on the period. The missing value will be blank.

[doma23]

How would I set up then the formula to show the percentage changes in the crosstab query (I suppose that's what you mean by pivot)?
There would only be one field in which I must write formula, and that's Period (which would be shown as column), what do I write to compare different periods?

Query view:

Result:

Edit: SOLVED
I solved it by making another query and bringing Crosstab query to it, which then showed periods "1H11" and "1Q11" as Fields.
It was quite simple after that.