The following is Informix behavior:
informix@zeus:/usr/informix72> dbaccess - -
> create database nulltest in datadbs;
Database created.
> create table t1 (a int4, b char(2), c char(2));
201: A syntax error has occurred.
Error in line 1
Near character position 20
> create table t1 (a int, b char(2), c char(2));
Table created.
> insert into t1 (a,c) values (1,'x');
1 row(s) inserted.
> insert into t1 (a,c) values (2,'x');
1 row(s) inserted.
> insert into t1 (a,c) values (3,'z');
1 row(s) inserted.
> insert into t1 (a,c) values (2,'x');
1 row(s) inserted.
> select * from t1;
a b c
1 x
2 x
3 z
2 x
4 row(s) retrieved.
> select b,c,sum(a) from t1 group by b,c;
b c (sum)
x 5
z 3
2 row(s) retrieved.
> select b,c,sum(a) from t1 group by b,c order by c;
b c (sum)
x 5
z 3
2 row(s) retrieved.
>
Andreas