You can easily get the average of values in a table in RethinkDB:
r.db('test')
 .table('salaries')
 .avg('salary')
If you want to do a GROUP BY as well, you’ll need to do a little more effort:
r.db('test')
 .table('salaries')
 .group('role')
 .avg('salary')
 .ungroup()
 .map(
    {Title: r.row('group'), 
     Amount: r.row('reduction')})
| 
 group 
 | 
 reduction 
 | 
|
| 
 1 
 | 
 null 
 | 
 80600 
 | 
| 
 2 
 | 
 Designer 
 | 
 72333.33333333333 
 | 
| 
 3 
 | 
 Manager 
 | 
 106250 
 |