Unique constraint on 2 columns, but in normal and reverse order - sql-server-2008

Question

Couldn't find any answer, so I'm writing this open question. I'm curious if is there any possibility to create such UNIQUE constraint on 2 columns in SQL Server 2008 table, that "normal" and "reverse" duplicates wouldn't be allowed.
Example:
ID1 = 10, ID2 = 20 -- existing row
Trying to add a pair of values:
ID1 = 10, ID2 = 20 -- not allowed because of the UNIQUE key
ID1 = 20, ID2 = 10 -- allowed
The second row will be inserted (of course it's not a duplicate). And that's the issue. Can any key/constraint/whatever be set on a table to disallow above insertion? I.e. something using an expression instead of list of columns? For now I use a trigger which checks for such "duplicates", but I just wonder if is there any simpler solution.
Thanks,
Peter P.

Answer 1

CREATE TABLE dbo.test
(ID1 int , ID2 int ,
CONSTRAINT ID_UK UNIQUE(ID1,ID2),
)
GO
IF EXISTS (SELECT name FROM sysobjects
WHERE name = 'check_val' AND type = 'TR')
DROP TRIGGER check_val
GO
CREATE TRIGGER check_val
ON dbo.test
FOR INSERT, UPDATE
AS
if exists ( select i.ID1 ,i.ID2 from inserted i inner join dbo.test t
on t.ID2=i.ID1 and t.ID1=i.ID2 )
RAISERROR ('duplicate values',
16, 1)
ROLLBACK TRANSACTION
GO
insert dbo.test
select 10,20
union
select 20,10

Answer 2

I just needed the exact same thing (just a couple years later)
I decided to go with a check constraint that requires ID1 to be less than ID2
i know that's kinda of hacky, which is why i'm not convinced it's better than the trigger.
now when i insert data, ID1 has to be the smaller else the check constraint fails, that coupled with the unique constraint ensures only one instance of the combination exists.
I'm not anti-trigger, just prefer to not use them unless i really need to
--create your table
create table dbo.test
(
ID1 int not null,
ID2 int not null
)
go
--create a unique constraint the prevents duplicate of id1 and id2
create unique index test_ID1_ID2_uindex
on test (ID1, ID2)
go
--create a check that ensures id1 is less than id2, disallowing something like Id1=20, id2=10
ALTER TABLE dbo.Test
ADD CONSTRAINT CHK_Test_ID1_Less_ID2 CHECK (ID1<ID2);
GO
--this will fail
insert dbo.test
select 10,20
union
select 20,10