Complex multi-column unique constraint - sql-server

Question

I have a table with 10 columns but only care about 3 columns for this. Imagine my table looks like this:
CREATE TABLE MyTable ( RowID int IDENTITY(1,1), UserID int, NodeID int, RoleID int )
What I need is a constraint that enforces the following: UserID and RoleID need to be unique for each NodeID (i.e. a user cannot have the same role in multiple nodes). In other words I want to allow
INSERT MyTable (UserID, NodeID, RoleID) SELECT 1, 1, 1
but not allow
INSERT MyTable (UserID, NodeID, RoleID) SELECT 1, 2, 1
if the first insert has occurred because that would result in a user having a role in multiple nodes.
Hopefully this is simple and I'm just making it more complex than it needs to be in my brain.

Answer 1

Since your constraint depends on data in other rows, this rules out a filtered index. IMO a viable option could be a trigger. Such a trigger could look like something like this:
CREATE TRIGGER dbo.MyTrigger ON dbo.Q1
AFTER INSERT, UPDATE
AS
DECLARE #userId INT, #Id INT, #roleId INT, #exists INT;
SELECT TOP 1
#userId = userID
,#roleId = roleID
,#Id = Id
FROM inserted;
SELECT TOP 1
#exists = Id
FROM Q1
WHERE userId = #userId
AND roleID = #roleID AND Id<> #Id;
IF ISNULL(#exists, 0) > 0
BEGIN
-- you would want to either undo the action here when you use an 'after' trigger
-- because as the name implies ... the after means the record is allready inserted/updated
RAISERROR ('No way we would allow this.', 16, 1);
END
-- else
-- begin
-- another alternative would be to use a instead of trigger, which means the record
-- has not been inserted or updated and since that type of trigger runs the trigger 'instead of'
-- updating or inserting the record you would need to do that yourself. Pick your poison ...
-- end
GO

Answer 2

An unique index should enforce your requirements
CREATE UNIQUE NONCLUSTERED INDEX [idx_Unique] ON [dbo].[MyTable]
(
[UserID] ASC,
[NodeID] ASC,
[RoleID] ASC
)
From the comments I suppose you will need two unique indices
CREATE UNIQUE NONCLUSTERED INDEX [idx_User_Node] ON [dbo].[MyTable]
(
[UserID] ASC,
[NodeID] ASC
)
GO
CREATE UNIQUE NONCLUSTERED INDEX [idx_User_Role] ON [dbo].[MyTable]
(
[UserID] ASC,
[RoleID] ASC
)