SQL Server, How to set auto increment after creating a table without data loss?

Question

I have a table table1 in SQL server 2008 and it has records in it.

I want the primary key table1_Sno column to be an auto-incrementing col

Answer 1

SQL Server: How to set auto-increment on a table with rows in it:

This strategy physically copies the rows around twice which can take a much longer time if the table you are copying is very large.

You could save out your data, drop and rebuild the table with the auto-increment and primary key, then load the data back in.

I'll walk you through with an example:

Step 1, create table foobar (without primary key or auto-increment):

CREATE TABLE foobar(
    id int NOT NULL,
    name nchar(100) NOT NULL,
)

Step 2, insert some rows

insert into foobar values(1, 'one');
insert into foobar values(2, 'two');
insert into foobar values(3, 'three');

Step 3, copy out foobar data into a temp table:

select * into temp_foobar from foobar

Step 4, drop table foobar:

drop table foobar;

Step 5, recreate your table with the primary key and auto-increment properties:

CREATE TABLE foobar(
    id int primary key IDENTITY(1, 1) NOT NULL,
    name nchar(100) NOT NULL,
)

Step 6, insert your data from temp table back into foobar

SET IDENTITY_INSERT temp_foobar ON
INSERT into foobar (id, name) select id, name from temp_foobar;

Step 7, drop your temp table, and check to see if it worked:

drop table temp_foobar;
select * from foobar;

You should get this, and when you inspect the foobar table, the id column is auto-increment of 1 and id is a primary key:

1    one
2    two
3    three