Let's say I have the following

```
Col1         Col2
1            A      
1            A
1            A
1            B
1            B
1            B
1            B
2            A
2            A
2            B
3            A
3            A
3            A
3            A
3            A
3            A

```

What i need to do is write a query that determines the number of records for each combination of Col1 and Col2 and divide 1 by that number and assign that value to a new column (Col3), however I also need to update records in some cases so when summing the new column it always adds to one for each combination of Col1 and Col2.

So, in the first step I would end up with

```
Col1         Col2          NumberofRows
1            A             3
1            B             4
2            A             2
2            B             1
3            A             6

```

which when dividing by 1 produces

```
Col1         Col2         Col3
1            A            0.33
1            A            0.33
1            A            0.33
1            B            0.25
1            B            0.25
1            B            0.25
1            B            0.25
2            A            0.5
2            A            0.5
2            B            1
3            A            0.17
3            A            0.17
3            A            0.17
3            A            0.17
3            A            0.17
3            A            0.17

```

However when summing Col3 where Col1 = 1 and Col2 = A we end up with 0.99 instead of 1 for obvious reasons. Similarly, summing Col3 where Col1=3 and Col2=A we end up with 1.02.

How can I do this?