Table constraint where a column is all null or all not null for a group of values

Hi all,

I've put in validation logic into the RPG maintenance application but I'm
afraid of someone entering invalid data using DFU, SQL, Excel or some other
method.

I'd like to know if it's possible to put some business logic into a table
constraint or what's the best method. I'm thinking an update trigger would
be my second choice.

Table: SIZEGRP
---------------------------------------------------
group CHAR(4) not null
size NUMERIC(4,0) not null
preference NUMERIC(2,0)

Primary key (group, size)
Unique (group, preference)
Check (preference > 0)

Sample data
---------------------
0001 11 -
0001 12 -
0001 13 -
0002 11 1
0002 12 2
0002 13 3

Business rule: Preferences in a group must either be all null or all not
null.
Based on this rule, the table should not allow either of these two rows:
0001 14 1
0002 14 -

Can this be done in a constraint?

Yours truly,

Glenn Gundermann
Email: glenn.gundermann@xxxxxxxxx
Work: (416) 675-9200 ext. 89224
Cell: (416) 317-3144