Sum over usage, and difference with group

1. sum over usage

Sum (col1) over (partition by col2 order by col3)

The above function can be understood as: grouping by col2 (partition), sorting by col3 (order), and continuous addition (sum)

Table a has the following content:
B C D
02 02 1
02 03 2
02 04 3
02 05 4
02 01 5
02 06 6
02 07 7
02 03 5
02 02 12
02 01 2
02 01 23

Run: SELECT B, c, d, SUM (d) OVER (partition by B, c ORDER BY d) e FROM

Expected result:

B C E
02 01 2
02 01 7
02 01 30
02 02 1
02 02 13
02 03 2
02 03 7
02 04 3
02 05 4
02 06 6
02 07 7

We can see that the values of column E are accumulate and.

2. Differences from group

Same Table a: select B, c, sum (d) e from a group by B, c

B C E
02 01 30
02 02 13
02 03 7
02 04 3
02 05 4
02 06 6
02 07 7

The difference is obvious.