Does SQL Server guarantee sequential inserting of an identity column?

Question

In other words, is the following \"cursoring\" approach guaranteed to work:

1. retrieve rows from DB
2. save the largest ID from the returned records for later,

Answer 1

The only thing that SQL Server guarantees is that your IDENTITY column will always be incremented.

Things to consider though:

1. If a fail INSERT occurs, the IDENTITY column will get incremented anyhow;
2. If a rollback occurs, the IDENTITY column will not return to its previous value;

Which explains why SQL Server doesn't guarantee sequential INDENTITY.

There is a way to reset an IDENTITY column like so using the DBCC command. But before doing so, please consider the following:

1. Ensure your IDENTITY column is not referenced by any other table, as your foreign keys could be not updated with it, so big troubles ahead;
2. You might use the SET IDENTITY_INSERT ON/OFF instruction so that you may manually specify the IDENTITY while INSERTing a row (never forget to turn it on afterward).

An IDENTITY column is one of the most important element never to be changed in DBRMs.

Here is a link that should help you: Understanding IDENTITY columns

EDIT: What you seem to do shall work as the IDENTITY column from LastMax will always increment for each INSERTed row. So:

1. Selecting rows from data table;
2. Saving LastMax state;
3. Selecting rows where Id > LastMax.

3) will only select rows where the IDENTITY column will be greater than LastMax, so inserted since LastMax has been saved.