How Do You Tell What Next Identity Column Will Be?

问题:

Is there a tsql query to tell what SQL server identity column value it expects to use for the next row insert?

Edited to add:

I deleted and recreated a table with

[personID] [int] IDENTITY(1,1) NOT NULL

as part of my CREATE TABLE command. I've also attempted to reseed identity columns while removing all information in that table and that hasn't always worked. It got me to wondering if there was a way to see what SQL expected to use for your next identity column number.

回答1:

No, there isn't any guaranteed way (although you can certainly find out what the next value might be, another command might go and use it before you can make any use of it). The only guaranteed value you can retrieve is the previously inserted identity value through SCOPE_IDENTITY() (which will return the identity value last generated for the current scope).

It's questionable what purpose why one would need to know the value before (when using an automatically incremented seeded identity column).

If you need to know the value before, then I recommend generating the ids yourself. You can do this with an ids table keyed on the table name, or, if you have scalability concerns (and you are using transactions) you can have an id table for each table that needs an id which would have the id to be inserted (and subsequently incremented).

Or, you could use a GUID, and you would be able to easily generate these on the client side before sending it to your database.

回答2:

You probably want to use SCOPE_IDENTITY not @@IDENTITY to restrict it to the identity value in the current scope. This avoids getting new identity values inserted by triggers into other tables and not the table you just inserted into.

But you can calculate what the next identity value is

SELECT IDENT_CURRENT('mytable') + IDENT_INCR('mytable') FROM mytable

The problem is you aren't guaranteed that is the value. You'd have to have a lock such that other inserts are denied on the table when running it to ensure the value is accurate. Also after you run out of 32 bit integers I don't know what the logic is. I don't know whether it rolls over or fails.

Edit: I just tested this (see below for SQL) and it doesn't return the correct value when there is no data. And reseeding with DBCC CHECKIDENT ('tablename', RESEED, 200) actually resulted in the next value being 201 not 200.

CREATE TABLE willtest (myid integer IDENTITY(1,1), myvalue varchar(255))

SELECT IDENT_CURRENT('willtest') + IDENT_INCR('willtest')

INSERT INTO willtest (myvalue)
VALUES ('1')
INSERT INTO willtest (myvalue)
VALUES ('2')
INSERT INTO willtest (myvalue)
VALUES ('3')
INSERT INTO willtest (myvalue)
VALUES ('4')
INSERT INTO willtest (myvalue)
VALUES ('5')
INSERT INTO willtest (myvalue)
VALUES ('6')
INSERT INTO willtest (myvalue)
VALUES ('7')
INSERT INTO willtest (myvalue)
VALUES ('8')

SELECT IDENT_CURRENT('willtest') + IDENT_INCR('willtest')

DBCC CHECKIDENT ('willtest', RESEED, 200)

SELECT IDENT_CURRENT('willtest') + IDENT_INCR('willtest')

INSERT INTO willtest (myvalue)
VALUES ('200')
INSERT INTO willtest (myvalue)
VALUES ('201')
INSERT INTO willtest (myvalue)
VALUES ('202')
INSERT INTO willtest (myvalue)
VALUES ('203')
INSERT INTO willtest (myvalue)
VALUES ('204')
INSERT INTO willtest (myvalue)
VALUES ('205')
INSERT INTO willtest (myvalue)
VALUES ('206')
INSERT INTO willtest (myvalue)
VALUES ('207')

SELECT IDENT_CURRENT('willtest') + IDENT_INCR('willtest')

SELECT * FROM willtest

DROP TABLE willtest

回答3:

This piece of sql will give you the next identity column value (there are probably many reasons not to repeat this snippet in production code)

declare @nextid int;
declare @previousid int;

begin tran

 insert into dbo.TestTable (Col1) values ('11');
 select @nextid = SCOPE_IDENTITY(); 

rollback tran

 select @previousid = @nextid -1
 DBCC CHECKIDENT('dbo.TestTable', RESEED, @previousid);
 select @nextid

this stackoverflow question gives some extra information - sql-identity-autonumber-is-incremented-even-with-a-transaction-rollback

回答4:

SELECT IDENT_CURRENT('mytable') + IDENT_INCR('mytable') FROM mytable

回答5:

Since you seed from 1 and increment by 1 (IDENTITY(1,1)), I'm wondering if you can create a procedure where you can set a variable like "Select @@IDENTITY + 1" or something like that.

回答6:

Use GUID columns for your primary keys. Unless you have billions of records and thousands of requests per second, you probably won't notice the performance difference. But unless you like spending far too much time dealing with stupid issues like this, you will notice the difference in your stress level and life expectancy.