SQL Server 2012, everything needs to be a SELECT statement with no procedures (platform limitations).

I have a table like:

| Field A | Field B | Number Field | Row Num |
----------------------------------------------
| A1      | X       | 19           | 1       |
| B1      |         | 20           | 2       |
|         | X       | 23           | 3       |
| A1      |         | 45           | 4       |
|         | Y       | 13           | 5       |
| C1      | Z       | 23           | 6       |
| C2      | Z       | 24           | 7       |
|         | Z       | 12           | 8       |
| C2      |         | 4            | 9       |
| C2      | J       | 3            | 10      |
| C4      | J       | 6            | 11      |
| C1      | J       | 5            | 12      | 

And I want a result like:

| Field A | Field B | Number Field Sum |
----------------------------------------
| A1      | X       | 87 (rows 1, 3, 4)
| B1      |         | 20 (row 2)
|         | Y       | 13 (row 5)
| C1      | Z       | 77 (rows 6-12)

For rows 6-7, I don’t care which combination of Field A and Field B is in the result set, only that all of the rows that are associated to each other are aggregated together.

I only need the sum or any other aggregate function, I don’t need text that says ‘(rows 6-12)’, that was just for explanation purposes.