Aggregate Function with Non-Aggregate Function In Select Command - sql

Question

I am exercising coding in MS.SQL Server. I have a small table below:
INV_NUM INV_AMOUNT
------- ----------
8000 235
8001 312
8002 528
8003 194
8004 619
I would like to write the query for all the invoices that will show:
the invoice number,
the invoice amount,
the average invoice amount, and
the difference between the average invoice amount and the actual invoice amount.
My code below:
SELECT I.INV_NUM,I.INV_AMOUNT,AVG(I.INV_AMOUNT) as AVERAGE,(AVG(INV_AMOUNT)-I.INV_AMOUNT) as DIFFER
FROM INVOICE I
GROUP BY I.INV_NUM,I.INV_AMOUNT;
The result in MS SQL did not generate the avg value, instead it shows INV_AMOUNT value in each row.
The result is :
INV_NUM INV_AMOUNT Average Differ
--------- ------------------------------------
8003 194 194 0
8000 235 235 0
8001 312 312 0
8002 528 528 0
8004 619 619 0
--------------- -------------------------------
I found if I have only one row in the table, it works fine.
Why this happened? Thanks!

Answer 1

I guess you want something like:
select
inv_num,
inv_amount,
avg(inv_amount) over() as average,
avg(inv_amount) over() - inv_amount as diff
from invoice
Your query is grouping your rows as separate groups. Instead you need to use a window function over a single group (using OVER).
Result:
inv_num inv_amount average diff
------- ---------- ------- ----
8000 235 377 142
8001 312 377 65
8002 528 377 -151
8003 194 377 183
8004 619 377 -242

Answer 2

You can use different solutions. As The Impaler mentioned you can use window functions. For diving deeper into this way of implementation you should read some tutorials about that.
Another way is using an CTE. You will first select the average invoice amount as own query. Afterwards you will use it in your "main" query. Looks like that:
WITH avg_inv AS
(
select avg(inv_amount) average
)
select
inv_num,
inv_amount,
(select average from avg_inv) as average,
(select average from avg_inv) - inv_amount as diff
from invoice;