How to properly increment an int identity in SQL Server if the Identity Increment isn't set?

Question

When I use SQL Server with an int based identity column, I always use the auto increment. But I'm needing to insert into an existing table that doesn't have the property set, and I'm wondering what is the best practices way of doing this.

The naive approach would be to query the data, increment it, and then use that for an insert.

For example:

INSERT INTO myTable
(
  ID,
  Data
)
Values
(
  (SELECT MAX(ID) FROM myTable) + 1,
  "My actual data."
)

But I'm unsure if this is actually the best approach. Namely, since this is all one statement, is there a risk for a heavily utilized system to have another row inserted between selecting the MAX(ID) and the insert.

Edit: Sql Server version 2012.

Edit 2: To specify, I'm looking for a DML solution as I do not have DDL rights to modify the table.

Answer 1

The best option is to use the SEQUENCE object, introduced in 2012. Since it is an independent object, you don't run the risk of querying it at the same time and retrieving the same value - it'll always provide the next in the chain.

Set the object with a specific start and increment value, then call it to get the next value desired. One of the biggest benefits here is that you can also specify a number of values to be cached for faster retrieval if you're working with rapid inserts.

CREATE SEQUENCE seq_obj
    START WITH 1050
    INCREMENT BY 1
    CACHE 1000
GO

Then to get the next ID value, use:

SELECT NEXT VALUE FOR seq_obj

Which will provide 1050, then 1051, and so on and so forth.

MS Docs for full information on other properties.

Answer 2

Given the answers above. If this is a one-off bulk insert, I'd create a temporary table with an identity column. Insert the data to it. And then select the data and insert to your target table.

You could set the identity start value, or start at 1 and add a fixed value when you do the final insert.

I can't check atmo but a believe data writer privileges should allow you to create your own #tables

Answer 3

I'm putting this as an answer because it's too long as a comment. It makes a lot of assumptions, but this could be one method. It's definitely a bit hokey but should work fairly reliably without creating objects in your target database. I think a temp table or other staging area may be required depending on what's doing the bulk import for you, but here is my thoughts.

1) Figure out how many records you are going to insert (bulk insert into a temp table?).

2) Get current MAX(ID) from Target Table

3) Add a dummy record to target table with MAX(ID) + 1 + RecordCount. NOTE: this assumes that the other inserts going on in this table are using MAX(ID) + 1

4) Make sure that nothing inserted between your now reserved range. This should avoid concurrency issues.

5) Assign values using MAX(ID) + Row_Number to your temp table or during bulk or whatever.

6) Insert into target table specifying the ID values. Making sure to overwrite your dummy record

Answer 4

Certainly, it will work:

INSERT INTO myTable
(
  ID,
  Data
)
SELECT MAX(ID)+1, "My actual data" FROM myTable;