Specify More Than One Column After compute

In a query, listing more than one column after the by keyword breaks a group into subgroups, and applies the specified row aggregate to each level of grouping.

For example, this query finds the sum of the prices of psychology books from each publisher:
select type, pub_id, price 
from titles 
where type = "psychology" 
order by type, pub_id, price 
compute sum(price) by type, pub_id 
type        pub_id  price 
----------- ------- -------------
psychology    0736           7.00 
psychology    0736           7.99 
psychology    0736          10.95 
psychology    0736          19.99 
Compute Result:
---------------
          45.93

type        pub_id  price 
----------- ------- -------------
psychology    0877          21.59 
 
Compute Result:
---------------
          21.59
 
(7 rows affected)