Mysql query to sum a column from a table where another two columns are unique when combined together

I have a Mysql table where it combines three tables. This table stores some statistics for each combination of the three foreign keys.

eg:

1)A Files table

columns-

id - primary key
name
number_of_lines_of_text - the stats field

2)A Folders table has a one to many relationship with files table

columns-

id - primary key
name

3)A Folder_Groups table has a many to many relationship with the folders table

columns-

id - primary key
name

Finally the c开发者_如何学Combination table ordered_file_stats

columns-

folder_group_id - foreign key from 3 above(can repeat multiple times)
folder_id - foreign key from from 2 above(can repeat muliple times)
file_id - foreign key from 1 above, unique but only a subset of the records from 1

I need to select and sum the number_of_lines_of_text field of the Files table for each unique combination of the folder_group_id and the folder_id in the ordered_file_stats table.

Which may look like

folder_group_id   folder_id  file_id
     1                2         1
     1                2         2
     1                3         9
     2                1         7
     2                1         8

How do I write the select statement to obtain the stats?

---

select ofs.folder_group_id, ofs.folder_id, sum( Files.number_of_line_of_text) 
from ordered_file_stats ofs
join Files on Files.id = ofs.file_id
group by folder_group_id, folder_id