How To Evaluate Data In Multi Rows

I am trying to evaluate dates within my sub queries for A,B,C,D. If there is a count > 0 in the 'A' category, I need to evaluate if the servicedates in B,C,D are within 30 days of 'A'. How would I code this within my current code?

select
customernumber,
SUM(CASE WHEN category = 'A' then 1 else 0 end) 'A',
SUM(CASE WHEN category = 'B' then 1 else 0 end) 'B',
SUM(CASE WHEN category = 'C' then 1 else 0 end) 'C',
SUM(CASE WHEN category = 'D' then 1 else 0 end) 'D'

[Code] ....

My BEFORE the case statement:

Customernumber A B C D SERVICEDATE
123456789 1 0 0 0 2012-02-07
123456789 0 0 1 0 2012-02-28
123456789 0 0 1 0 2012-03-06
123456789 0 1 0 0 2012-12-17

My output after case statement:

Customernumber A B C D
123456789 1 1 2 0

What I need my output to look like after case statement:

Customernumber A B C D Service w/in 3 mnths
123456789 1 1 2 0 2