Inserting NULL in an nvarchar fails in MSAccess - sql-server

Question

I'm experiencing something a bit strange.
I have a table on SQL Server 2008, say StockEvent that contains a Description field defined as nVarchar(MAX).
The field is set to be Nullable, has no default value and no index on it.
That table is linked into an Access 2007 application, but if I explicitly insert a NULL into the field, I'm systematically getting:
Run-time Error '3155' ODBC--insert on a linked table 'StockEvent' failed.
So the following bits of code in Access both reproduce the error:
Public Sub testinsertDAO()
Dim db As DAO.Database
Dim rs As DAO.Recordset
Set db = CurrentDb
Set rs = db.OpenRecordset("StockEvent", _
dbOpenDynaset, _
dbSeeChanges + dbFailOnError)
rs.AddNew
rs!Description = Null
rs.Update
rs.Close
Set rs = Nothing
Set db = Nothing
End Sub
Public Sub testinsertSQL()
Dim db As DAO.Database
Set db = CurrentDb
db.Execute "INSERT INTO StockEvent (Description) VALUES (NULL);", _
dbSeeChanges
Set db = Nothing
End Sub
However, if I do the same thing from the SQL Server Management Studio, I get no error and the record is correctly inserted:
INSERT INTO StockEvent (Description) VALUES (NULL);
It doesn't appear to be machine-specific: I tried on 3 different SQL Server installations and 2 different PCs and the results are consistent.
I initially though that the problem may be in my Access application somewhere, but I isolated the code above into its own Access database, with that unique table linked to it and the results are consistent.
So, is there some known issue with Access, or ODBC and inserting NULL values to nvarchar fields?
Update.
Thanks for the answers so far.
Still no luck understanding why though ;-(
I tried with an even smaller set of assumptions: I created a new database in SQL Server with a single table StockEvent defined as such:
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[StockEvent](
[ID] [int] IDENTITY(1,1) NOT NULL,
[Description] [nvarchar](max) NULL
) ON [PRIMARY]
GO
Then linked that table though ODBC into the test Access 2007 application.
That application contains no forms, nothing except the exact 2 subroutines above.
If I click on the linked table, I can edit data and add new records in datasheet mode.
Works fine.
If I try any of the 2 subs to insert a record, they fail with the 3155 error message.
(The table is closed and not referenced anywhere else and the edit datasheet is closed.)
If I try the SQL insert query in SQL Server Management Studio, it works fine.
Now for the interesting bit:
It seems that anything as big or bigger than nvarchar(256), including nvarchar(MAX) will fail.
Anything with on or below nvarchar(255) works.
It's like Access was considering nvarchar as a simple string and not a memo if its size is larger than 255.
Even stranger, is that varchar(MAX) (wihout the n) actually works!
What I find annoying is that Microsoft's own converter from Access to SQL Server 2008 converts Memo fields into nvarchar(MAX), so I would expect this to work.
The problem now is that I need nvarchar as I'm dealing with Unicode...

Answer 1

OK, I may have found a related answer: Ms Access linking table with nvarchar(max).
I tried using the standard SQL Server driver instead of the SQL Server Native Client driver and nvarchar(MAX) works as expected with that older driver.
It really annoys me that this seems to be a long-standing, unfixed, bug.
There is no valid reason why nvarchar should be erroneously interpreted as a string by one driver and as a memo when using another.
In both cases, they appear as memo when looking a the datatype under the table design view in Access.
If someone has any more information, please leave it on this page. I'm sure others will be glad to find it.

Answer 2

That should be legal syntax. Is it possible that the field you are try to give a null value is linked to other fields that don't allow null values?

Answer 3

Potential concurrency problem... Is the record open by another instance of Access on the same or a different machine, or does a form bound to the table have the record open in the same instance of Access on the same machine?
Renaud, try putting something in one of the other fields when you do the insert.
Also, try inserting an empty string ("") instead of a null.

Answer 4

Renaud,
Did you try running a SQL Profiler trace? If you look at the Errors and Warnings category it should kick out an error if your insert failed as a result of a SQL Server constraint.
If you don't see any errors, you can safely assume that the problem is in your application.
Also, are you sure you're actually connected to SQL Server? Is CurrentDB not the same variable you're using in your Access test loop?

Answer 5

i got annother issue (here my post: link text
In some very rare cases an error arises when saving a row with a changed memo field - same construct explained in my former post but driving sql2000-servers and it's appropriate odbc-driver (SQL SERVER).
The only weired fix is: to expand the table structure on sql-server with a column of datatype [timestamp] and refresh the odbc-links. That works and releases the show-stopper in this column on this one row ...
Maybe this info can help someone - for me it's history in going further to odbc with sql2008 in changing the datatypes [text] to [varchar(max)].