With QGIS >=2.16, it is possible to use aggregates in expressions, such as sum, average… etc.
But when I use it to create a new virtual field using the field calculator,
the newly created field value is null for all of the layer's entities.
An example here : 
Am I doing it wrong ? Or it is a bug ?
I tried to change the field type with no effect.
Note that it works when you create a non-virtual field.
Best Answer
Fisrt, your field needs to be quoted.
Now look this example, the first one is to add the sum of all the field in each row; the second one, is to sum by row in a specific field.
You have Integer or Real (decimal number) to set your field, depending on what you need to get.