sql - Fill In The Date Gaps With Date Table

Question

I have two tables.

An orders table with customer, and date. A date dimension table from a data warehouse.

The orders table does not contain activity for every date in a given month, but I need to return a result set that fills in the gaps with date and customer.

For Example, I need this:

Customer   Date
===============================
Cust1       1/15/2012   
Cust1       1/18/2012
Cust2       1/5/2012
Cust2       1/8/2012

To look like this:

Customer   Date
============================
Cust1       1/15/2012   
Cust1       1/16/2012   
Cust1       1/17/2012       
Cust1       1/18/2012
Cust2       1/5/2012
Cust2       1/6/2012
Cust2       1/7/2012
Cust2       1/8/2012

This seems like a left outer join, but it is not returning the expected results. Here is what I am using, but this is not returning every date from the date table as expected.

SELECT o.customer, 
       d.fulldate
FROM   datetable d 
       LEFT OUTER JOIN orders o 
                    ON d.fulldate = o.orderdate 
WHERE  d.calendaryear IN ( 2012 ); 

Answer 1

The problem is that you need all customers for all dates. When you do the left outer join, you are getting NULL for the customer field.

The following sets up a driver table by cross joining the customer names and dates:

SELECT driver.customer, driver.fulldate, o.amount 
FROM   (select d.fulldate, customer
        from datetable d cross join
             (select customer
              from orders
              where year(orderdate) in (2012)
             ) o
        where d.calendaryear IN ( 2012 )
       ) driver LEFT OUTER JOIN
       orders o 
       ON driver.fulldate = o.orderdate and
          driver.customer = o.customer;

Note that this version assumes that calendaryear is the same as year(orderdate).