Unique constraint on multiple columns when any columns are not null - sql-server

Question

I am trying to add a unique constraint on 2 columns that allows multiple (null, null), but doesn't allow multiple ("a", null). I wrote the following SQL statement but I get an error
Incorrect syntax near the keyword 'with'
SQL statement:
CREATE UNIQUE NONCLUSTERED INDEX [UQ]
ON [dbo].[MyTable] ([Column_A], [Column_B])
WHERE [Column_A] IS NOT NULL OR [Column_B] IS NOT NULL
WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY];
However in
WHERE [Column_A] IS NOT NULL OR [Column_B] IS NOT NULL
when I replace OR with AND, there is no syntax error anymore. But what I want is OR logic, not AND because using AND allows multiple ("a", null) entries.
So why is there an incorrect syntax error using OR? Thanks.

Answer 1

It's a bit clunky, but if the point is to enforce uniqueness (except for NULL/NULL) and you're happy having two indexes, you can do it via putting the NOT NULL filters on separate indexes e.g.,
CREATE UNIQUE NONCLUSTERED INDEX
[UQ_A] ON [dbo].[MyTable]
(
[Column_A],
[Column_B]
)
WHERE [Column_A] IS NOT NULL
GO
CREATE UNIQUE NONCLUSTERED INDEX
[UQ_B] ON [dbo].[MyTable]
(
[Column_A],
[Column_B]
)
WHERE [Column_B] IS NOT NULL
GO
While it's not necessary for the uniqueness check, you may want to change the order of the fields in the second index to potentially gain additional advantages of the index (depending on how these fields are used in your application - it may be better swapping them in the first index), e.g.,
CREATE UNIQUE NONCLUSTERED INDEX
[UQ_B] ON [dbo].[MyTable]
(
[Column_B],
[Column_A]
)
WHERE [Column_B] IS NOT NULL
GO

Answer 2

According to Martin Smith's comment, I think I can implement it by:
CREATE VIEW [dbo].[TableAView]
WITH SCHEMABINDING AS
SELECT
[ColumnA] = [ColumnA],
[ColumnB] = [ColumnB]
FROM [dbo].[TableA]
WHERE [ColumnA] IS NOT NULL OR [ColumnB] IS NOT NULL;
GO
CREATE UNIQUE CLUSTERED INDEX
[UQ] ON [dbo].[TableAView]
(
[ColumnA],
[ColumnB]
)
WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON)
ON [PRIMARY];
GO