从总计中查找每一行的平均值

Question

I have a sales table sale with following columns

sale_id | Department | gross_amount

I need to find the avg of sales in each department from the total sale

eg:- Table

sale_id     Department     gross_amount
  1            A             10
  2            B             30
  3            A             25
  4            c              5

Desired output

Department               Gross_amount     avg
  A                          35            50       --(35/70)*100
  B                          30            42.86    --(30/70)*100
  C                          5             7.14     --(5/70) *100

ie is dept_avg = (dept_total / total) *100 eg:- dept_total of A = 35 total = A+B+C = 35+30+5 =70

I am able to find up to Gross_amount

select Department ,sum(si.GrossPrice)  gross_amt
from Sale si
group by Department 
order by Department

For getting avg I tried follow

 select Department ,sum(si.GrossPrice) gross_amt,
    AVG(sum(si.GrossPrice)) avg
    from Sale si
    group by Department 
    order by Department

It is giving me an error

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

Also I am not sure i can get my expected avergage with the above query. How can I achieve the same.

Answer 1

Divide grouped sum by total sum:

SqlFiddleDemo

SELECT Department ,
     [Gross_amount] = SUM(gross_amount),
     [avg] = ROUND(CAST(SUM(gross_amount) AS DECIMAL(10,2))/
                   CAST((SELECT SUM(gross_amount) FROM tab) AS DECIMAL(10,2)) * 100
             ,2)
FROM tab
GROUP BY Department 
ORDER BY Department

Answer 2

If you want the proportion of the total for each department out of the overall total, I would suggest using window functions:

SELECT Department, SUM(gross_amount) as gross_amount
       SUM(gross_amount)*100.0 / SUM(SUM(gross_amount)) over () as propotion
FROM tab
GROUP BY Department 
ORDER BY Department;