Question : TSQL:  How to plug Nulls with Lookup values in same column and limited keys

I have a query that returns a data set which looks like this:

ID   DATE            GOAL     CURRENT
1    2007-01-01  NULL      18
1    2007-02-01  NULL      18
1    2007-03-01  16          18
1    2007-04-01  NULL      18
1    2007-05-01  NULL      18
1    2007-06-01  17          18

In other words, the goal value is set quarterly, and resides only in the last period of the quarter.   I need to plug this value into the prior two periods.  The only thing I have to key off of is the ID (and maybe the year and month dateparts).  I can't do anything about the source application responsible for the original insert.

Is there a way to do this with block-sql?  Prefer avoiding the dreaded cursor, but at this point I'm desperate.  It's less than 25k rows.

Answer : TSQL:  How to plug Nulls with Lookup values in same column and limited keys

SELECT id, date,
 SUM(goal) OVER (PARTITION BY DATEDIFF(MONTH, '20070101',date)/3) goal,
 [current]
FROM tbl;