DBCC CHECKIDENT Sets Identity to 0

Question

I'm using this code to reset the identity on a table:

DBCC CHECKIDENT('TableName', RESEED, 0) 

This works fine most of the time, with the first insert I do inserting 1 into the Id column. However, if I drop the DB and recreate it (using scripts I've written) and then call DBCC CHECKIDENT, the first item inserted will have an ID of 0.

Any ideas?

EDIT: After researching I found out I didn't read the documentation properly - "The current identity value is set to the new_reseed_value. If no rows have been inserted to the table since it was created, the first row inserted after executing DBCC CHECKIDENT will use new_reseed_value as the identity. Otherwise, the next row inserted will use new_reseed_value + 1. "

Answer 1

You are right in what you write in the edit of your question.

After running DBCC CHECKIDENT('TableName', RESEED, 0):
- Newly created tables will start with identity 0
- Existing tables will continue with identity 1

The solution is in the script below, it's sort of a poor-mans-truncate :)

-- Remove all records from the Table DELETE FROM TableName  -- Use sys.identity_columns to see if there was a last known identity value -- for the Table. If there was one, the Table is not new and needs a reset IF EXISTS (SELECT * FROM sys.identity_columns WHERE OBJECT_NAME(OBJECT_ID) = 'TableName' AND last_value IS NOT NULL)      DBCC CHECKIDENT (TableName, RESEED, 0); 

Answer 2

As you pointed out in your question it is a documented behavior. I still find it strange though. I use to repopulate the test database and even though I do not rely on the values of identity fields it was a bit of annoying to have different values when populating the database for the first time from scratch and after removing all data and populating again.

A possible solution is to use truncate to clean the table instead of delete. But then you need to drop all the constraints and recreate them afterwards

In that way it always behaves as a newly created table and there is no need to call DBCC CHECKIDENT. The first identity value will be the one specified in the table definition and it will be the same no matter if you insert the data for the first time or for the N-th