SQL unit value and cumulative sum by groups

SQL unit value and cumulative sum by groups

I'm still new to SQL and am kind of stumped on this one. I would greatly
appreciate any help or advice. I have a table with a value column and an
ID column that I then order by the value column in descending order. i.e:
Value | ID
12 | A
09 | A
08 | B
08 | C
07 | A
06 | B
03 | B
01 | C
I am trying to do two things:
1) For each row, calculate the percent of the total sum for its respective
ID. Row 1 would be 12/(12+9+7), row 2: 3/(12+9+7), row 3: 8/(8+6+3), etc.
2) Calculate the running total of the percentage calculated in (1) for
each ID. Essentially just a cumulative sum for each ID.
The output would look something like this (sorry for the poor formatting)
Value | ID | UnitValue | RunningTotal
12 | A | 0.43 | 0.43
09 | A | 0.32 | 0.75
08 | B | 0.47 | 0.47
08 | C | 0.89 | 0.89
07 | A | 0.25 | 1.00
06 | B | 0.35 | 0.82
03 | B | 0.18 | 1.00
01 | C | 0.11 | 1.00