In MS SQL Server, is there a way to “atomically” increment a column being used as a counter?

Question

Assuming a Read Committed Snapshot transaction isolation setting, is the following statement \"atomic\" in the sense that you won\'t ever \"lose\" a concurrent increment?

Answer 1

There is at heart only one transaction, the outermost one. The inner transactions are more like checkpoints within a transaction. Isolation levels affect only sibling outermost transactions, not parent/child related transactions.

The counter will be incremented by two. The following yields one row with a value of (Num = 3). (I opened up SMSS and pointed it to a local SQL Server 2008 Express instance. I have a database named Playground for testing stuff.)

use Playground

drop table C
create table C (
    Num int not null)

insert into C (Num) values (1)

begin tran X
    update C set Num = Num + 1
    begin tran Y
        update C set Num = Num + 1
    commit tran Y
commit tran X

select * from C