How can SCOPE_IDENTITY return null when @@IDENTITY does not?

sql-server, sql-server-2005

Solution

here is one example of how SCOPE_IDENTITY() will be null but @@IDENTITY will have a value:

insert into a table with no identity, that table has an insert trigger that then inserts into a history table with an identity. SCOPE_IDENTITY() will be null (no identity in the local scope), but @@IDENTITY will report the identity from the trigger.

FYI, there is a known bug with SCOPE_IDENTITY(): https://connect.microsoft.com/SQLServer/feedback/ViewFeedback.aspx?FeedbackID=328811

Your best bet with identities is to use OUTPUT INTO, it can capture a set of IDs and is not subject to the SCOPE_IDENTITY() bug:

declare @x table (tableID int identity not null primary key, datavalue varchar(10))
declare @y table (tableID int, datavalue varchar(10))

INSERT INTO @x values ('aaaa')
INSERT INTO @x values ('bbbb')
INSERT INTO @x values ('cccc')
INSERT INTO @x values ('dddd')
INSERT INTO @x values ('eeee')


INSERT INTO @x
    (datavalue)
    OUTPUT INSERTED.tableID, INSERTED.datavalue     --<<<<OUTPUT INTO SYNTAX
    INTO @y                                         --<<<<OUTPUT INTO SYNTAX
SELECT
    'value='+CONVERT(varchar(5),dt.NewValue)
    FROM (SELECT id as NewValue from sysobjects where id<20) dt
    ORDER BY dt.NewValue


select * from @x
select * from @y

Problem

After executing an insert, I either select `SCOPE_IDENTITY` or `@@IDENTITY`. `SCOPE_IDENTITY` returns null but `@@IDENTITY` does not. I don't understand how this is possible. Can you think of a reason why this happens?

Original source