Sorry for delayed response, Matt.

I looked up the book that covers Standard SQL and it says..

" SELECT select_list
FROM table_list
[ where condition ]
GROUP BY group_by_list
In most SQL dialects, every item in the GROUP BY list must be a column name
(not an expression) and must be equivalent to or a subset of select list items - you can make groups only out of things you select . "
.....
Restrictions. GROUP BY looks straightforward, but it's caused its share of headaches for SQL users.
Here,for example, is a query that seems reasonable but won't work on most systems:
SQL"
select pub_id, type, count(type)
from titles
group by pub_id

Since the table is divided into sets by publishers (group by pub_id) and there are three publishers, the query must returen no more than three rows, and each select list item must have a single value for the set. Unfortunately, there is more than one type per publisher, so most systems find the query impossible to answer. You could solve this problem by adding type to the GROUP BY clause, ..

select pub_id, type, count(type)
from titles
group by pub_id, type
"
You'd better think You must identify every select list in group by list no matter what the DBMS is.

Hope this help.