Add autoincrement to existing PK

The way I understand your question is that you have an existing table with a column that has up until now been populated with manual values, and now you want to (1) make this column an IDENTITY column, and (2) make sure that the IDENTITY starts from the most recent value in the existing rows.

First off, some test data to play with:

CREATE TABLE dbo.ident_test (
    id    int NOT NULL,
    xyz   varchar(10) NOT NULL,
    CONSTRAINT PK_ident_test PRIMARY KEY CLUSTERED (id)
);

INSERT INTO dbo.ident_test (id, xyz)
VALUES (1, 'test'),
       (2, 'test'),
       (5, 'test'),
       (6, 'test'),
       (10, 'test'),
       (18, 'test'),
       (19, 'test'),
       (20, 'test');

The goal is to make the table's primary key column, id, an IDENTITY column that will start at 21 for the next record that gets inserted. For this example, the column xyz represents all of the table's other columns.

Before you do anything, please read the warnings at the bottom of this post.

First off, in case something goes wrong:

BEGIN TRANSACTION;

Now, let's add a temporary work column, id_temp and set that column to the existing id column's values:

ALTER TABLE dbo.ident_test ADD id_temp int NULL;
UPDATE dbo.ident_test SET id_temp=id;

Next, we need to drop the existing id column (you can't just "add" an IDENTITY to an existing column, you have to create the column as an IDENTITY). The primary key also has to go, because the column depends on it.

ALTER TABLE dbo.ident_test DROP CONSTRAINT PK_ident_test;
ALTER TABLE dbo.ident_test DROP COLUMN id;

... and add the column again, this time as an IDENTITY, along with the primary key:

ALTER TABLE dbo.ident_test ADD id int IDENTITY(1, 1) NOT NULL;
ALTER TABLE dbo.ident_test ADD CONSTRAINT PK_ident_test PRIMARY KEY CLUSTERED (id);

Here's where it gets interesting. You can enable IDENTITY_INSERT on the table, which means that you can manually define the values of an IDENTITY column when you're inserting new rows (not updating existing rows, though).

SET IDENTITY_INSERT dbo.ident_test ON;

With that set, DELETE all the rows in the table, but the rows that you're deleting are OUTPUT right into the very same table - but with specific values for the id column (from the backup column).

DELETE FROM dbo.ident_test
OUTPUT deleted.id_temp AS id, deleted.xyz
INTO dbo.ident_test (id, xyz);

Once, done, turn IDENTITY_INSERT back off again.

SET IDENTITY_INSERT dbo.ident_test OFF;

Drop the temporary column that we added:

ALTER TABLE dbo.ident_test DROP COLUMN id_temp;

And finally, reseed the IDENTITY column, so the next record's id will resume after the highest existing number in the id column:

DECLARE @maxid int;
SELECT @maxid=MAX(id) FROM dbo.ident_test;
DBCC CHECKIDENT ("dbo.ident_test", RESEED, @maxid)

Checking the example table, the highest id number is 20.

SELECT * FROM dbo.ident_test;

Add another row and check its new IDENTITY:

INSERT INTO dbo.ident_test (xyz) VALUES ('New row');
SELECT * FROM dbo.ident_test;

In the example, the new row will have id=21. Finally, if you're happy, commit the transaction:

COMMIT TRANSACTION;

Important

This is not a trivial operation, and it carries quite a few risks that you should be aware of.

Do this in a dedicated test environment. Have backups. :)
I like to use BEGIN/COMMIT TRANSACTION because it prevents other processes from messing with the table while you're in the middle of changing it, and it gives you the possibility to roll everything back if something goes wrong. However, any other process that tries to access your table before you've committed your transaction will end up waiting. This may be pretty bad if you have a large table and/or you are on a production environment.
OUTPUT .. INTO won't work if your target table has foreign key constraints or any of a number of other features that I can't remember off the top of my head. You could instead off-load the data into a temporary table instead, and then insert it back into the original table. You might be able to use partition switching (even if you don't use partitions).
Run these statements one by one, not as a batch or in a stored procedure.
Try to think of other things that may depend on the id column that you're dropping and re-creating. Any indexes will have to be dropped and re-created (like we did with the primary key). Remember to script every index and constraint that you will need to recreate beforehand.
Disable any INSERT and DELETE triggers on the table.

If re-creating the table is an option:

If re-creating the table is an option for you, everything is a lot simpler:

Create the empty table, with the id column as an IDENTITY,
Set IDENTITY_INSERT ON for the table,
Populate the table,
Set IDENTITY_INSERT OFF, and
Reseed the identity.

Using UPDATE, DELETE or INSERT to move data can take quite a lot of time and use resources (IO) on both data and log files/disks. It is possible to avoid filling the transaction log with potentially lots of records while working on a big table: Using partition switching, only the metadata is changed.

There is no data movement involved and this is therefore performed really quickly (almost instantaneous).

Sample table

The question does not show the original table DDL. The following DDL will be used as an example in this answer:

CREATE TABLE dbo.idT(
    id int not null
    , uid uniqueidentifier not null
    , name varchar(50)
);
ALTER TABLE dbo.idT ADD CONSTRAINT PK_idT PRIMARY KEY CLUSTERED(id);

Half a dozen dummy random ids from 0 to 15 are added with this query:

WITH ids(n) AS(
    SELECT x1.n+x2.n*4
    FROM (values(0), (3)) as x1(n)
    CROSS JOIN (values(0), (2), (3)) as x2(n)
)
INSERT INTO idt(id, uid, name)
SELECT n, NEWID(), NEWID() 
FROM ids

Example data in `IdT`

id  uid                                     name
0   65533096-5007-43EA-88AD-D6776B3B94FA    6A69D4F2-D682-4168-A92F-4CD2E2DBC21D
3   CE87F1ED-BE1A-4F2D-8D62-E1ECA822D35B    AF0524D9-0DBB-41E1-883B-003CB4E4F012
8   34A1DBFD-4F92-4F34-9F04-4CDC824AB15A    02B4BDA4-D515-4262-9031-0BE496AC24CE
11  51606C95-9DE8-4C30-B23B-F915EEA41156    93258103-9C22-4F9C-85CF-712ED0FB3CE6
12  CEC80431-0513-4751-A250-0EB3390DACAB    2DA6B8AF-3EBC-42B3-A76C-028716E24661
15  5037EA83-286F-4EBC-AD7C-E237B570C1FF    095E51E9-8C38-4104-858F-D14AA810A550

New table with `IDENTITY(0, 1)`

The only problem with idT is the lack of the IDENTITY(0, 1) property on id. A new table with a similar structure and IDENTITY(0, 1) is created:

CREATE TABLE dbo.idT_Switch(
    id int identity(0, 1) not null
    , uid uniqueidentifier not null
    , name varchar(50)
);
ALTER TABLE dbo.idT_Switch ADD CONSTRAINT PK_idT_Switch PRIMARY KEY CLUSTERED(id);

Aside from IDENTITY(0, 1), idT_Switch is identical to idT.

Foreign Keys

Foreign keys on idT must be removed to allow this technique to be used.

Partition Switch

The idT and idT_Switch tables have a compatible structure. Instead of using DELETE, UPDATE and INSERT statements to move rows from idT to idT_Switch or on idT itself, ALTER TABLE ... SWITCH can be used:

ALTER TABLE dbo.idT
SWITCH TO dbo.idT_Switch;

The single 'partition' of PK_idT (the whole table) is moved to PK_idT_Switch (and vice versa). idT now contains 0 rows and idT_Switch contains 6 rows.

You can find the full list of source and destination compatibility requirements here:

Transferring Data Efficiently by Using Partition Switching

Note this use of SWITCH does not require Enterprise Edition, because there is no explicit partitioning. An unpartitioned table is considered to be a table with a single partition from SQL Server 2005 onward.

Replace `idT`

idT is now empty and useless and can be dropped:

DROP TABLE idT;

idT_Switch can be renamed and will replace the old idT table:

EXECUTE sys.sp_rename
    @objname = N'dbo.idT_Switch',
    @newname = N'idT', -- note lack of schema prefix
    @objtype = 'OBJECT';

Foreign keys

Foreign keys can be added again to the new idT table. Anything else previously removed from idT to make the tables compatible for switching will also need to be redone.

Reseed

SELECT IDENT_CURRENT( 'dbo.idT');

This command returns 0. Table idT contains 6 rows with MAX(id) = 15. DBCC CHECKIDENT ( table_name ) can be used:

DBCC CHECKIDENT ('dbo.idT');

Because 15 is bigger than 0, it will automatically reseed without looking for MAX(id):

If the current identity value for a table is less than the maximum identity value stored in the identity column, it is reset using the maximum value in the identity column. See the 'Exceptions' section that follows.

IDENT_CURRENT now returns 15.

Test and add data

A simple INSERT statement:

INSERT INTO idT(uid, name) SELECT NEWID(), NEWID();

Adds this row:

id  uid                                     name
16  B395D692-5D7B-4DFA-9971-A1497B8357A1    FF210D9E-4027-479C-B5D8-057E77FAF378

The id column is now using the identity and the newly inserted value is indeed 16 (15+1).

More information

There is a related question and answer with more background on the SWITCH technique here:

Why is removing the Identity property on a column not supported

If you want to start with a new identity value you need to reseed your identity. Have a look at the documentation for CHECKIDENT

DBCC CHECKIDENT (yourtable, reseed, starting point)

Add autoincrement to existing PK

Sample table

Example data in `IdT`

New table with `IDENTITY(0, 1)`

Foreign Keys

Partition Switch

Replace `idT`

Foreign keys

Reseed

Test and add data

More information

Tags:

Sql Server

Primary Key

Auto Increment

Identity

Related

Recent Posts

Add autoincrement to existing PK

Sample table

Example data in IdT

New table with IDENTITY(0, 1)

Foreign Keys

Partition Switch

Replace idT

Foreign keys

Reseed

Test and add data

More information

Tags:

Sql Server

Primary Key

Auto Increment

Identity

Related

Example data in `IdT`

New table with `IDENTITY(0, 1)`

Replace `idT`