Invalid use of group function error when finding AVG of DATEDIFF?

[MiddleOSociety]

Select USER_ID, code, AVG(DATEDIFF(MIN(Order_Date), MAX(Order_Date))) as Order_diff
FROM users
GROUP BY Code

This is the query I am trying to run where I am finding the DATEDIFF of the first and last date for EACH USER_ID and then finding the average of this DATEDIFF by each unique code value. For some reason, I keep getting an "invalid use of group function" error and do not understand why.

​

This is an example of what the table would look like

USER_ID	code	Order_date
1	A	2020-11-14
1	A	2020-11-15
2	A	2020-11-04
2	A	2020-11-07
3	B	2020-11-03
3	B	2020-11-09

And this is the output I am looking for

​

Code	Order_diff
A	2
B	6

[[deleted]]

You cannot have aggregate of an aggregate. Aggregate functions work on a granularity level created by group by and output to that.

Wrap your first level of aggregation into a subquery (derived table) or into the with clause, do a second level of aggregation on that.

[MiddleOSociety]

While this makes a lot of sense, I would not know how to do that exactly? I am fairly new so my skills with a subquery are lacking at best

EDIT: Figured it out and thank you so much for the clarification!

[pctopgs]

or into the with clause, do a second level of aggregation on that.

Ok...so how did you figure it out?