TSQL Datediff(dd,MAX(p_effdt),CURRENT_TIMESTAMP)

Hi,

OBJECTIVE: for a group of clients, report the average number of days in which they have been enrolled in a given program.

PROBLEM: the clients are often dropped and later re-enrolled in our programs.  So, I want to use only the most recent enrollment date for each client.  And then calculate an average number of days enrolled.

The data looks like this:

c_id	p_effdt	        p_lapdt
123456	12/19/2013	NULL
234567	11/10/2003	12/23/2003
345678	11/25/2010	NULL
345678	12/16/2010	4/29/2010
456789	11/11/2004	11/30/2004
456789	11/13/2003	12/29/2003

Of course, this: Datediff(dd,MAX(p_effdt),CURRENT_TIMESTAMP)

results in the error: Cannot perform an aggregate function on an expression containing an aggregate or a subquery.

DECLARE @LastProgramEffectiveDate DATETIME
SET @LastProgramEffectiveDate = MAX(program.p_effdt)

SELECT AVG(Datediff(dd,@LastProgramEffectiveDate,CURRENT_TIMESTAMP))
FROM events INNER JOIN program ON events.e_case_no = program.c_id

So that SQL query is not the way to go.  What is the best way to accomplish the above defined objective?

Thank you.

Alpineer8