Question

I am trying to get the average number of months difference between two dates:

select client_id, 
        avg(12*(year(MAX(received_date))-year(min(received_date))) 
        + MONTH(MAX(received_date))-MONTH(min(received_date)))
        from tmpTwoAccessions
        group by CLIENT_ID,PATIENT_ID

I am getting this message:

Cannot perform an aggregate function on an expression containing an aggregate or a subquery.

Can you please advise me on what am I doing incorrectly here?
Thanks so much for your guidance.

Answer 1

You do the difference between the MAX and MIN in a sub-query, and then average that in the outer query.

SELECT client_id, AVG(delta_date)
  FROM (SELECT client_id, patient_id,
               12 * (YEAR(MAX(received_date))-YEAR(MIN(received_date))) 
               + MONTH(MAX(received_date)) - MONTH(MIN(received_date))) AS delta_date
          FROM tmpTwoAccessions
         GROUP BY client_id, patient_id
       ) AS x
 GROUP BY client_id;

I’m not convinced your date difference expression is optimal, but I’ve not tried changing it.

---

Yurly Rozhovetskiy suggested the expression:

DATEDIFF(MONTH, MIN(received_date), MAX(received_date))

which looks good to me. The revised query becomes:

SELECT client_id, AVG(delta_date)
  FROM (SELECT client_id, patient_id,
               DATEDIFF(MONTH, MIN(received_date), MAX(received_date)) AS delta_date
          FROM tmpTwoAccessions
         GROUP BY client_id, patient_id
       ) AS x
 GROUP BY client_id;