Reseeding Identity Column to Original Value - sql

Question

Is there any way to reseed an identity column to it's original value? Currently I have this IDENTITY(1,1) as my table definition. I've tried reseeding it using this method:
DBCC CHECKIDENT (Table1, reseed, 1)
But I noticed, that SQL actually reseeds it to current seed + the new seed value, so when I insert new rows it starts the auto-increment count at 2 instead of 1. I've also tried making the new reseed value 0, but then all the rows I insert after that end up being 1 and the column doesn't auto-increment anymore.
By the way, if it helps, I need this column to restart its auto-increment count at zero because I need for the count to restart every month. I have trigger after insert set up for this, but the reseeding is giving me some trouble.

Answer 1

The following statement:
DBCC CHECKIDENT('YourTableName', RESEED, 1);
Tells the database something akin to:
"Hey database! The highest identity value in the YourTableName table is #1, so the next inserted record should use identity value #2."
This explains is why your table starts with identity value 2 when you RESEED it using your statement.
More specifically Microsoft's help for DBCC CHECKIDENT() indicates:
Current identity value is set to the new_reseed_value. If no rows have been inserted to the table since it was created, or all rows have been removed by using the TRUNCATE TABLE statement, the first row inserted after you run DBCC CHECKIDENT uses new_reseed_value as the identity. Otherwise, the next row inserted uses new_reseed_value + the current increment value.
If the table is not empty, setting the identity value to a number less than the maximum value in the identity column can result in one of the following conditions:
If a PRIMARY KEY or UNIQUE constraint exists on the identity column, error message 2627 will be generated on later insert operations into the table because the generated identity value will conflict with existing values.
If a PRIMARY KEY or UNIQUE constraint does not exist, later insert operations will result in duplicate identity values.
It sounds like you want to change your table's initial starting seed value from 1 to 0. With this in mind, you need to modify the table's definition to use:
IDENTITY(0,1);
I hope this helps. Good luck.

Answer 2

Not really an answer, but too long for a comment and does cover the question.
I've also tried making the new reseed value 0, but then all the rows I insert after that end up being 1 and the column doesn't auto-increment anymore.
I can't reproduce this behaviour:
IF OBJECT_ID(N'dbo.T', 'U') IS NOT NULL
DROP TABLE dbo.T;
CREATE TABLE dbo.T(ID INT IDENTITY(1,1) NOT NULL);
INSERT dbo.T OUTPUT inserted.* DEFAULT VALUES;
-- OUTPUTS 1
DELETE dbo.T;
DBCC CHECKIDENT ('dbo.T', RESEED, 0);
INSERT dbo.T OUTPUT inserted.* DEFAULT VALUES;
-- OUTPUTS 1, I.E. RESEED TO 0 HAS WORKED FINE
INSERT dbo.T OUTPUT inserted.* DEFAULT VALUES;
-- OUTPUTS 2, SHOWS INCREMENT IS WORKING AS EXPECTED
IF OBJECT_ID(N'dbo.T', 'U') IS NOT NULL
DROP TABLE dbo.T;
It is worth noting however that for an empty table the identity will start at 0 if you use 0 as the RESEED value, i.e.
TRUNCATE TABLE dbo.T;
DBCC CHECKIDENT ('dbo.T', RESEED, 0);
INSERT dbo.T OUTPUT inserted.* DEFAULT VALUES;
Will output 0 as the ID inserted.

Answer 3

I believe the only way to do this would be to select all your data into a 2nd table.
Then you can issue the command truncate table Table1.
This will reset the identity column.
Then you re-insert all your data back from temporary holding into the main table (without inserting the old identity).
Example...
NB: run each code block one at a time, checking counts along the way, don't run the whole thing -- and I've left the "dangerous" bits commented out just in case
/** select all columns but NOT the id into a temp location (dont use a # table) **/
SELECT col1, col2, col3, col4, col5
INTO tmpTable
FROM Table1
/** before truncation, double check counts in both match!! **/
-- SELECT COUNT(2) FROM tmpTable
-- SELECT COUNT(2) from Table1
-- TRUNCATE TABLE Table1
INSERT INTO Table1
SELECT col1, col2, col3, col4, col5
FROM tmpTable
/** before you drop temp table, make sure Table1 is ok! **/
-- SELECT COUNT(2) FROM tmpTable
-- SELECT COUNT(2) from Table1
-- DROP TABLE tmpTable
Here is a quick test to demonstrate -- I insert 8 rows, delete one, and re-insert. They are correctly re-numbered.
IF OBJECT_ID('DBO.TEST') IS NOT NULL DROP TABLE TEST
IF OBJECT_ID('DBO.TMP') IS NOT NULL DROP TABLE TMP
CREATE TABLE TEST ([ID] INT IDENTITY(1,1), [SOMESTRING] NVARCHAR(50))
INSERT INTO TEST([SOMESTRING])
SELECT 'HI' UNION
SELECT 'THESE' UNION
SELECT 'ARE' UNION
SELECT 'SOME' UNION
SELECT 'STRINGS' UNION
SELECT 'JUST' UNION
SELECT 'TO' UNION
SELECT 'DEOMONSTRATE'
SELECT *
FROM TEST
DELETE FROM TEST
WHERE SOMESTRING='SOME'
SELECT SOMESTRING
INTO TMP
FROM TEST
TRUNCATE TABLE TEST
INSERT INTO TEST
SELECT SOMESTRING
FROM TMP
SELECT * FROM TEST