>I have two tables - we'll call them Customer and Bill. A customer can have zero or many bills, and it is linked by the CustomerID. What I want to do is pull a customer by CustomerID and also pull their last bill only if it is within 30 days old. If there isn't a bill that fits that criteria, I want null value.
>
>Can I do this with a JOIN? Anybody have any advice on how to do the filtering in the JOIN statement?

Something like that

DECLARE @WantedDate datetime
SET @WantedDate = '20070412'

SELECT Customer.*, -- All from customer table
       Tbl2.*
FROM Customer
LEFT JOIN (SELECT CustomerId, (other fields from BILL)
                  FROM Bill
                  INNER JOIN (SELECT CustomerId, MAX(BillDate) AS BillDate
                                     FROM Bill
                                     WHERE BillDate BETWEEN DateAdd(dd,-30, @WantedDate) AND @WantedDate
                                     GROUP BY CustomerId) Tbl1
                 ON Bill.CustomerId = Tbl1.CustomerId AND 
                    Bill.BillDate   = Tbl1.BillDate) Tbl2
ON Customer.CustomerId = Tbl2.CustomerId

not tested