Set auto id to primary key column

Question

I have a table and primary key is already set to that table and now I want that column to be autoincrement. Table has many records. Is it possible? or which one is fastest way to do that?

Answer 1

I think you have to make some effort for this as you cannot create identity column on existing column. However you may have a workaround for this like first try this to add a new column having identity field:

ALTER TABLE dbo.Table_name
   ADD ID INT IDENTITY

and then make your ID as primary key like this:

ALTER TABLE dbo.Table_name
   ADD CONSTRAINT PK_YourTable
   PRIMARY KEY(ID)

And yes you have to remove the old dependencies before performing the above steps like this:

ALTER TABLE Table_name
DROP CONSTRAINT PK_Table1_Col1

EDIT:-

From the source:

We can use ALTER TABLE...SWITCH to work around this by only modifying metadata. See Books Online for restrictions on using the SWITCH method presented below. The process is practically instant even for the largest tables.

USE tempdb;
GO
-- A table with an identity column
CREATE TABLE dbo.Source (row_id INTEGER IDENTITY PRIMARY KEY NOT NULL, data SQL_VARIANT NULL);
GO
-- Some sample data
INSERT dbo.Source (data) 
VALUES (CONVERT(SQL_VARIANT, 4)), 
        (CONVERT(SQL_VARIANT, 'X')), 
        (CONVERT(SQL_VARIANT, {d '2009-11-07'})),
        (CONVERT(SQL_VARIANT, N'áéíóú'));
GO
-- Remove the identity property
BEGIN TRY;
    -- All or nothing
    BEGIN TRANSACTION;

    -- A table with the same structure as the one with the identity column,
    -- but without the identity property
    CREATE TABLE dbo.Destination (row_id INTEGER PRIMARY KEY NOT NULL, data SQL_VARIANT NULL);

    -- Metadata switch
    ALTER TABLE dbo.Source SWITCH TO dbo.Destination;

    -- Drop the old object, which now contains no data
    DROP TABLE dbo.Source;

    -- Rename the new object to make it look like the old one
    EXECUTE sp_rename N'dbo.Destination', N'Source', 'OBJECT';

    -- Success
    COMMIT TRANSACTION;
END TRY
BEGIN CATCH
    -- Bugger!
    IF XACT_STATE() <> 0 ROLLBACK TRANSACTION;
    PRINT ERROR_MESSAGE();
END CATCH;
GO

-- Test the the identity property has indeed gone
INSERT dbo.Source (row_id, data)
VALUES (5, CONVERT(SQL_VARIANT, N'This works!'))

SELECT row_id,
        data
FROM    dbo.Source;
GO

-- Tidy up
DROP TABLE dbo.Source;