group by or compute in a view definition

This restriction applies to all columns in views that contain aggregate values—that is, views whose definition includes a group by or compute clause. Here is a view defined with a group by clause and the rows seen through it:

create view categories (category, average_price) 
as select type, avg(price) 
from titles 
group by type 
select * from categories 
category         average_price 
-------------    ------------- 
UNDECIDED                 NULL 
business                 13.73 
mod_cook                 11.49 
popular_comp             21.48 
psychology               13.50 
trad_cook                15.96 
 
(6 rows affected) 

It does not make sense to insert rows into the view categories. To what group of underlying rows would an inserted row belong? Updates on the average_price column are not allowed because there is no way to know from any value you might enter there how the underlying prices should be changed.