Beginning Microsoft SQL Server 2008 ... - S3 Tech Training

How It Works
What we’re doing in this script is seeing what happens if we depend on @@IDENTITY directly rather
than moving the value to a safe place. When we execute the preceding script, everything’s going to work
just fine until the final INSERT. That final statement is trying to make use of @@IDENTITY directly, but
the preceding INSERT statement has already changed the value in @@IDENTITY. Because that statement
is on a table with no identity column, the value in @@IDENTITY is set to NULL. Because we can’t have a
NULL value in our primary key, the last INSERT fails:
(1 row(s) affected)
The value we got originally from @@IDENTITY was 1
The value currently in @@IDENTITY is 1
(1 row(s) affected)
The value we got originally from @@IDENTITY was 1
The value currently in @@IDENTITY is NULL
Msg 515, Level 16, State 2, Line 41
Cannot insert the value NULL into column ‘IDcol’, table
‘Accounting.dbo.TestChild2’; column does not allow nulls. INSERT fails.
The statement has been terminated.
If we make just one little change (to save the original @@IDENTITY value):
/*****************************************
** This script illustrates how the identity
** value gets lost as soon as another INSERT
** happens
****************************************** */
DECLARE @Ident int; -- This will be a holding variable
/* We’ll use it to show how we can
** move values from system functions
** into a safe place.
*/
INSERT INTO TestIdent
DEFAULT VALUES;
SET @Ident = @@IDENTITY;
PRINT ‘The value we got originally from @@IDENTITY was ‘ +
CONVERT(varchar(2),@Ident);
PRINT ‘The value currently in @@IDENTITY is ‘ + CONVERT(varchar(2),@@IDENTITY);
/* On this first INSERT using @@IDENTITY, we’re going to get lucky.
** We’ll get a proper value because there is nothing between our
** original INSERT and this one. You’ll see that on the INSERT that
** will follow after this one, we won’t be so lucky anymore. */
INSERT INTO TestChild1
VALUES
(@@IDENTITY);
PRINT ‘The value we got originally from @@IDENTITY was ‘ +
CONVERT(varchar(2),@Ident);
IF (SELECT @@IDENTITY) IS NULL
PRINT ‘The value currently in @@IDENTITY is NULL’;
Chapter 11: Writing Scripts and Batches
333