SQL Server add auto increment primary key to existing table
As the title, I have an existing table which is already populated with 150000 records. I have added an Id column (which is currently null).
I'm assuming I can run a query to fill this column with incremental numbers, and then set as primary key and turn on auto increment. Is this the correct way to proceed? And if so, how do I fill the initial numbers?
No - you have to do it the other way around: add it right from the get go as INT IDENTITY - it will be filled with identity values when you do this:
ALTER TABLE dbo.YourTable
ADD ID INT IDENTITY
and then you can make it the primary key:
ALTER TABLE dbo.YourTable
ADD CONSTRAINT PK_YourTable
PRIMARY KEY(ID)
or if you prefer to do all in one step:
ALTER TABLE dbo.YourTable
ADD ID INT IDENTITY
CONSTRAINT PK_YourTable PRIMARY KEY CLUSTERED
You can't "turn on" the IDENTITY: it's a table rebuild.
If you don't care about the number order, you'd add the column, NOT NULL, with IDENTITY in one go. 150k rows isn't a lot.
If you need to preserve some number order, then add the numbers accordingly. Then use the SSMS table designer to set the IDENTITY property. This allows you to generate a script which will do the column drop/add/keep numbers/reseed for you.
I had this issue, but couldn't use an identity column (for various reasons). I settled on this:
DECLARE @id INT
SET @id = 0
UPDATE table SET @id = id = @id + 1
Borrowed from here.
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