How do I add the identity property to an existing column in SQL Server

Question

In SQL Server (in my case, 2005) how can I add the identity property to an existing table column using T-SQL?

Something like:

alter table tblFoo 
            


        

Answer 1

There is no direct way of doing this except:

A) through SQL i.e.:

-- make sure you have the correct CREATE TABLE script ready with IDENTITY

SELECT * INTO abcTable_copy FROM abcTable

DROP TABLE abcTable

CREATE TABLE abcTable -- this time with the IDENTITY column

SET IDENTITY_INSERT abcTable ON

INSERT INTO abcTable (..specify all columns!) FROM (..specify all columns!) abcTable_copy

SET INDENTITY_INSERT abcTable OFF

DROP TABLE abcTable_copy 

-- I would suggest to verify the contents of both tables 
-- before dropping the copy table

B) Through MSSMS which will do exactly the same in the background but will less fat-fingering.

• In the MSSMS Object Explorer right click the table you need to modify
• Select "design" Select the column you'd like to add IDENTITY to
• Change the identity setting from NO -> YES (possibly seed)
• Ctr+S the table

This will drop and recreate the table with all original data in it. If you get a warning:

Go to MSSMS Tools -> Options -> Designers -> Table and database Designers and uncheck the option "Prevent saving changes that require table re-creation"

Things to be careful about:

1. your DB has enough disk space before you do this
2. the DB is not in use (especially the table you are changing)
3. make sure to backup your DB before doing it
4. if the table has a lot of data (over 1G) try it somewhere else first before using in real DB