How can SCOPE_IDENTITY return null when @@IDENTITY does not?
After executing an insert, I either select SCOPE_IDENTITY
or @@I开发者_开发知识库DENTITY
.
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?
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
KM hit the nail on the head:
@@IDENTITY
gives you the last IDENTITY value inserted - no matter where in which table it was inserted (think triggers, e.g. into audit tables! Or even a cascade of triggers.....)SCOPE_IDENTITY()
gives you the last IDENTITY inserted in the scope of your statement, e.g. on the table(s) that your own, actual statement referenced (not those that might have been touched by a trigger)
SCOPE_IDENTITY will also return NULL when the insert is by sp_executesql as you are no longer in the scope of the INSERT!
I found this on MSDN:
The SCOPE_IDENTITY() function will return the null value if the function is invoked before any INSERT statements into an identity column occur in the scope.
You can read here: http://msdn.microsoft.com/en-us/library/ms190315.aspx
Your SQL code would be very helpful.
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