Why do I get multiple rows with Sum() and GroupBy

Question

When I sum a column, I get a single result of all rows summed up. When I group by on this summed column, I get returned multiple rows. Can someone explain what is going on? Is it secretly grouping all the rows with data that is the same and summing these all up first as separate rows?

Table Column Data is

9.75
2.25
8.25
8.25
7
8
5
10.5
5
10.5
10.5
10.25
10.5
10.5
5
5
0.5
5
5
0.5
5
5
0.5
5
5
0.5
5
5
0.5
5
5
0.5
5
5
0.5
5
5
0.5
5
5
0.5
5
5
0.5
1.5
5
1.75
0.5
5
5
0.5
1.5
5
1.75
0.5
5
5
0.5
5
5
0.5
5
5
0.5
5
5
0.5
5
5
0.5
5
5
0.5
5
5
0.5
5
5
0.5
1.75
5
5
0.5
5
10
10
10
10
5
0.5
5
5
0.5
1.75
5
1.5
0.5
5
5
0.5
5
5
0.5
NULL

The Result I get from "Group By column"

Select Sum(column) as [column]
From Table
Group By column  

(Result)
NULL
13.5
4.5
7
2.25
265
7
8
16.5
9.75
40
10.25
52.5

The result from just the Sum

Select Sum(column) as [column]
From Table

(result)
436.25

Answer 1

count , avg , and sum are all aggregate functions . They take a group of rows and aggregate them together into one value. count gives the number of rows, avg gives their mean, and sum sums them up.

By default all rows are aggregated together. group by allows you to split things up into more groups. When you group by column you say to aggregate together all the rows with the same value for column . So you're getting the sums of all rows with the same value for column .

You can see this by also selecting the column and the count.

select column, count(column), sum(column)
from table
group by column

You'll find that sum(column) is column times count(column) . For example.

column, count(column), sum(column)
1.5     3              4.5
8       1              8
10      4              40