How to ensure column uniqueness

Hi,
Coming from an Oracle background, I'm used to being able
to create a unique key on a column that allows many null
values, ie if a value exists it must be unique, otherwise
it can be null.
It appears as though SQL Server allows only 1 null value
in the same situation, ie create a unique constraint on a
column and once you try to insert a 2nd row with a null
value I get a constraint violation. [Thx to those that
replied to my last question on this]
I don't really want to write triggers testing for such
conditions. Is there some form of constraint that can do
this for me?
TIA,
SJTYour observations are correct. You can create a view conatining all rows but the NULL. Then create a
unique index (or possible a unique constraint) on that index.
--
Tibor Karaszi, SQL Server MVP
Archive at: http://groups.google.com/groups?oi=djq&as ugroup=microsoft.public.sqlserver
"SJT" <scott.taylor@.pwcs.com.au> wrote in message news:054601c36ad9$46dd1f90$a501280a@.phx.gbl...
> Hi,
> Coming from an Oracle background, I'm used to being able
> to create a unique key on a column that allows many null
> values, ie if a value exists it must be unique, otherwise
> it can be null.
> It appears as though SQL Server allows only 1 null value
> in the same situation, ie create a unique constraint on a
> column and once you try to insert a 2nd row with a null
> value I get a constraint violation. [Thx to those that
> replied to my last question on this]
> I don't really want to write triggers testing for such
> conditions. Is there some form of constraint that can do
> this for me?
>
> TIA,
> SJT|||You can use an indexed view to enforce uniqueness only for non-NULL values:
CREATE TABLE Sometable (keycol INTEGER PRIMARY KEY, colx INTEGER NULL)
GO
CREATE VIEW Sometable_Unique_Non_NULL
WITH SCHEMABINDING
AS SELECT colx FROM dbo.Sometable WHERE colx IS NOT NULL
GO
CREATE UNIQUE CLUSTERED INDEX uclcolx ON Sometable_Unique_Non_NULL (colx)
INSERT INTO Sometable VALUES (1,1)
INSERT INTO Sometable VALUES (2,NULL)
INSERT INTO Sometable VALUES (3,NULL)
--
David Portas
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