SQL unique constraint on either of 2 columns - sql

Question

I have a table in SQL that I would like to have a unique constraint so that any of two values cannot already exist.
for example if I have 2 columns, I would like it to not insert if the value in column B does not exist in column A or column B.
Is this possible and if so how is it done?
example:
Column A | Column B
--------------------
4 | 6
I would want any object that tries to insert 4 or 6 not to be allowed into the table

Answer 1

Trigger with ROLLBACK TRANSACTION is the way to go.
create trigger dbo.something after insert as
begin
if exists ( select * from inserted where ...check here if your data already exists... )
begin
rollback transaction
raiserror ('some message', 16, 1)
end
end

Answer 2

You can create a function which takes in these values & create a check constraint on it (referencing your functions return values) to your table.
create table t11 (Code int, code2 int)
create function fnCheckValues (#Val1 int, #Val2 int)
Returns int /*YOu can write a better implementation*/
as
Begin
DECLARE #CntRow int
IF(#Val1 IS NULL OR #Val2 IS NULL) RETURN 0
select #CntRow = count(*) from t11
where Code in (#Val1,#Val2 ) or Code2 in (#Val1,#Val2 )
RETURN #CntRow
End
GO
alter table t11 Add constraint CK_123 check ([dbo].[fnCheckValues]([Code],[code2])<=(1))

Answer 3

When one want to enforce a multi-row constraint that is not offered by the database engine, the obvious solution is use of a trigger or stored procedure. This often does not work because the database isolates the transactions the triggers and stored procedures run in, allowing violations in the presense of concurrency.
Instead turn the constraint into something that the database engine will enforce.
CREATE TABLE dbo.T (A INT, B INT)
GO
CREATE TABLE dbo.T_Constraint_Helper (ColumnName sysname PRIMARY KEY)
INSERT INTO dbo.T_Constraint_Helper (ColumnName)
VALUES ('A'), ('B')
GO
CREATE VIEW T_Constraint_VW
WITH SCHEMABINDING AS
SELECT CASE CH.ColumnName WHEN 'A' THEN T.A ELSE T.B END AS Value
FROM dbo.T
CROSS JOIN dbo.T_Constraint_Helper CH
GO
CREATE UNIQUE CLUSTERED INDEX FunnyConstraint_VW_UK ON dbo.T_Constraint_VW (Value)
GO
INSERT INTO T VALUES (1, 2)
-- works
INSERT INTO T VALUES (2, 3)
-- Msg 2601, Level 14, State 1, Line 1
-- Cannot insert duplicate key row in object 'dbo.T_Constraint_VW' with unique index 'T_Constraint_VW_UK'. The duplicate key value is (2).
INSERT INTO T VALUES (4, 4)
-- Msg 2601, Level 14, State 1, Line 1
-- Cannot insert duplicate key row in object 'dbo.T_Constraint_VW' with unique index 'T_Constraint_VW_UK'. The duplicate key value is (4).
INSERT INTO T VALUES (5, 6)
-- works