SQL SERVER – Add or Remove Identity Property on Column

This article contribution from one of my favorite SQL Expert Imran Mohammed. He is one man who has lots of ideas and helps people from all over the world with passion using this community as platform. His constant zeal to learn more about SQL Server keeps him engaging him to do new SQL Server related activity every time.

1. Adding Identity Property to an existing column in a table.

How difficult is it to add an Identity property to an existing column in a table? Is there any T-SQL that can perform this action?

For most, the answer to the above two questions is an absolute NO! There is no straightforward T-SQL like ALTER TABLE… or MODIFY COLUMN to add an Identity Property to an existing column in a table.

However, there is an easy way to accomplish this action. It can be done through SSMS.

Are you finding my answer difficult to believe? Let me explain.

Let’s first see what SSMS does in backend when you add Identity property on an existing column in any table.

Now, let’s create an example table for better understanding.

This table is vacant, with no records, as you can see in the following screenshot.

Take a look at the design of this table in SSMS.

Now let us make eid, an Identity column.

This is very easy.  All you have to do is just select Yes from the drop down list and you are done!

But before moving further let’s see what T-SQL SQL Server is using to make this change.

You will notice that T-SQL is used by SQL Server to make this change.

After you make the change for Identity property from No to Yes, on top in tools box, you will see Generate Change Script. This is the T-SQL Script that SQL Server will use to make this change.

Unfortunately, I cannot expand this dialogue box further to show you the complete script, so I have copied this script below.

/* To prevent any potential data loss issues, you should review this script in detail before running it outside the context of the database designer.*/
BEGIN TRANSACTION
SET QUOTED_IDENTIFIER ON
SET ARITHABORT ON
SET NUMERIC_ROUNDABORT OFF
SET CONCAT_NULL_YIELDS_NULL ON
SET ANSI_NULLS ON
SET ANSI_PADDING ON
SET ANSI_WARNINGS ON
COMMIT
BEGIN TRANSACTION
GO
CREATE TABLE dbo.Tmp_example1
(
eid INT NOT NULL IDENTITY (1, 1)
) ON [PRIMARY] GO
SET IDENTITY_INSERT dbo.Tmp_example1 ON
GO
IF EXISTS(SELECT * FROM dbo.example1)
EXEC('INSERT INTO dbo.Tmp_example1 (eid)
SELECT eid FROM dbo.example1 WITH (HOLDLOCK TABLOCKX)')
GO
SET IDENTITY_INSERT dbo.Tmp_example1 OFF
GO
DROP TABLE dbo.example1
GO
EXECUTE sp_rename N'dbo.Tmp_example1', N'example1', 'OBJECT'
GO
COMMIT

Let us understand above code in simple words:

Begin Transaction
Step 1: Create similar table structure as example1, say Tmp_Example1 with Identity Column.
Step 2: Set IDENTITY_INSERT ON on new table just created.Step 3: Insert all values from example1 to Tmp_Example1
Step 4: Set IDENTITY_INSERT OFF on Tmp_Example..
Step 5: Drop example1 (What is going on… dropping our original table???)
Step 6: Rename Tmp_Example1 to Example1.
Commit Transaction

What if you have millions and millions of records in your table?

Imagine how much time it would take to copy an entire table?

Just to make a column Identity do you really need to go through all these and take so much pain?

Certainly not!

Same is the case when you want to remove Identity column property from a column.

2. Removing Identity Property from an existing column in a table.

/* To prevent any potential data loss issues, you should review this script in detail before running it outside the context of the database designer.*/
BEGIN TRANSACTION
SET QUOTED_IDENTIFIER ON
SET ARITHABORT ON
SET NUMERIC_ROUNDABORT OFF
SET CONCAT_NULL_YIELDS_NULL ON
SET ANSI_NULLS ON
SET ANSI_PADDING ON
SET ANSI_WARNINGS ON
COMMIT
BEGIN TRANSACTION
GO
CREATE TABLE dbo.Tmp_example1
(
eid INT NOT NULL
) ON [PRIMARY] GO
IF EXISTS(SELECT * FROM dbo.example1)
EXEC('INSERT INTO dbo.Tmp_example1 (eid)
SELECT eid FROM dbo.example1 WITH (HOLDLOCK TABLOCKX)')
GO
DROP TABLE dbo.example1
GO
EXECUTE sp_rename N'dbo.Tmp_example1', N'example1', 'OBJECT'
GO
COMMIT

Once again, I would like to congratulate Imran Mohammed for this wonderful article.

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