1. How do I... Reseed a SQL Server identity column? | TechRepublic
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How do I... Reseed a SQL
Server identity column?
By Susan Harkins
July 15, 2008, 11:53 AM PDT
Microsoft SQL Server’s identity column generates sequential values for new
records using a seed value. The term seed refers to the internal value SQL
Server uses to generate the next value in the sequence. By default, an identity
column’s first value is 1 and each new value increments by one (1, 2, 3, 4, and
so on).
You can control the column’s first and subsequent values, by specifying those values when you
create the column. For instance, you might want to begin with a purchase order number of 1001 Data Masking
rather than 1.
Software
Once the table’s in production you can reset, or reseed the column. In other words, you can Secure your sensitive data with Grid-Tools'
change the column’s seed value at any time. For instance, you might reseed a column after Enterprise Data Masking
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deleting records or moving data to an archive table.
Multilingual WebCenter
The good news is that seeding and reseeding an identity column is easy and relatively safe, if you Translate Content Inside WebCenter Lingotek
do it correctly. It’s common to seed a new table or to reseed a production, but empty table. Collaborative Translation
However, as a general rule, it’s not a great idea to reseed an identity column in a table that lingotek.com/oracle
contains data without some serious checks and balances. SQL Server Training
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Throughout this article, I use SQL Server Express and Management Studio Express because both SSWUG.ORG Community Today!
are free and therefore available to all readers. However, the logic and instructions are the same in www.sswug.org
SQL Server.
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About identity
Developers and writers alike tend to refer to SQL Server’s identity column property as a data type.
It’s actually a column property. In purpose, it’s similar to Microsoft Access‘ AutoNumber column,
although SQL Server’s identity column is more flexible. However, there are a few restrictions:
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An identity column must be one of the following numeric data types: decimal, int, numeric,
smallint, bigint, or tinyint.
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An identity column can’t accept or store NULL.
Each table can contain only one identity column.
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When you create an identity column, you specify two values: Identity Seed and Identity Increment.
The seed value specifies the column’s first (or next) value. SQL Server adds the increment value
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2. How do I... Reseed a SQL Server identity column? | TechRepublic
to the last identity value to generate the next value, in sequence.
For instance, a seed value of 1 and an increment value of 2 will generate the values 1, 3, 5, 7, and
so on. By default, both the seed and incremental values are 1. Business rules often require a bit of
customization.
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Seeding
Using Management Studio is probably the easiest way to seed an identity column when you create
the table. In this case, it’s a simple matter of setting the appropriate property value, as you can
see in Figure A. You can also use Management Studio to reseed an existing identity column.
Figure A PHOTO GALLERY (1 of 15)
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'seven minutes of...
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Seed an identity column when you create it in Management Studio.
2 windows 7 won't shutdown and
If you create the table using code, you can easily seed the table’s identity column using the keeps switching on
Transact-SQL (T-SQL) CREATE TABLE statement in the following form:
2 can anyone suggest if any such
CREATE TABLE tablename
software exist with similar
( functionality?
columnname datatype identity [(seed, increment)
Ask a Question
[NOT FOR REPLICATION]],
[columnname ...]
) Hot Discussions View All
In the code, datatype is a numeric column. Both seed and increment are optional and the default
221 Should developers be sued for
value for both is 1.
security holes?
Figure B shows the result of using CREATE TABLE to create a table named Orders and setting
the OrderID column’s identity seed and increment values to 100 and 10, respectively. As you can 79 The sitting duck that is open
see, the first identity value is 100 and each subsequent value increases by 10. (You can decrease source
identity values by specify a negative value for increment.)
27 Five fast Windows desktop search
Figure B utilities
30 Is the death knell sounding for
traditional antivirus?
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3. How do I... Reseed a SQL Server identity column? | TechRepublic
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Use CREATE TABLE to seed an identity column.
Checking and reseeding
For instance, if you copy all the table’s records to an archive table and then delete all the records
in the source table, you might want to reseed the source table’s identity column, so you can
control the sequence. Use T-SQL’s DBCC CHECKIDENT as follows to reseed an identity column:
DBCC CHECKIDENT
(
tablename
[, [NORESEED | RESEED [, newreseedvalue]]]
)
[WITH NO_INFOMSGS]
Table A defines this statement’s optional parameters.
Table A: DBCC CHECKIDENT
Parameter Description
Returns the current identity value and the current maximum value of
NORESEED
the identity column, without reseeding. These values are usually (and
should be) the same.
Changes the current identity value, using the maximum value in the
RESEED
identity column, if the current identity value is less than the maximum
identity value stored in the identity column.
Specifies the new seed value when reseeding. If the table is empty,
the first identity value (after executing DBCC CHECKIDENT) will equal
newreseedvalue newreseedvalue. If the table contains data, the next identity value will
equal newreseedvalue + the current increment value (the default is 1).
This behavior is new to SQL Server 2005 (and remains in 2008). SQL
Server 2000 always increments the seed value.
WITH NO INFOMSGS
Suppresses all informational messages.
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4. How do I... Reseed a SQL Server identity column? | TechRepublic
Technically, DBCC CHECKIDENT checks and corrects an identity value. Simply put, use it to learn
the current identity value or to reseed an existing identity column.
Checks
Once a table’s in production, you might want to determine the seed value, as shown in Figure C.
Executing DBCC CHECKIDENT with no argument other than the table’s name returns the
column’s current identity value, which is the last value generated and stored in the table (see
Figure B).
Figure C
Return an identity column’s current and maximum values.
These two values will usually be the same. If they’re not, reset the identity value to avoid errors or
gaps in the sequence of values.
Balances
SQL Server won’t execute a statement that duplicates existing identity values if the identity column
is part of a primary key. Figure D shows what can happen when an identity column isn’t part of a
primary key. In this case, DBCC CHECKIDENT reseeds OrderID and accepts a new record.
However, the identity column generates a duplicate value. It’s important to remember that an
identity column does not guarantee uniqueness.
Figure D
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5. How do I... Reseed a SQL Server identity column? | TechRepublic
What can happen when an identity column isn’t part of a primary key.
When the identity column is part of a primary key, SQL Server protects the validity of the existing
data by returning a violation error, as shown in Figure E. Now, the question is, did SQL Server
reset the seed value or not — it did. You can reseed the column to a value that has the potential
to create a duplicate, but SQL Server won’t let you enter the record. That’s why you must be
careful to check existing identity values before reseeding.
Figure E
A violation error.
When reseeding identity values, keep the following behaviors in mind:
Setting the newreseed value below existing values will eventually generate an error if the
column is part of a primary key.
Use the exact seed value when running DBCC CHECKIDENT against a new table that has
never contained data. For instance, a newreseed value of 10 will generate an initial identity
value 10.
When resetting an empty table (after deleting records), the identity column will generate
newreseed + 1. If the next identity value should be 1, use a newreseed value of 0.
Deleting records from a table using DELETE doesn’t reset an identity column; using
TRUNCATE TABLE does. TRUNCATE TABLE is usually faster than DELETE, but in this case,
not necessarily the best choice for the job, unless you mean to reset the identity column.
Reseeding basics
Reseeding an identity column is a common task, but you must know the basics or you might get
into trouble. Specifically, an identity column can generate duplicate values. Knowing when and
why is the key to keeping an identity column in check.
Susan Sales Harkins is an independent consultant and the author of several articles and books on
database technologies. Her most recent book is “Mastering Microsoft SQL Server 2005 Express,”
with Mike Gunderloy, published by Sybex. Other collaborations with Mike Gunderloy are
“Automating Microsoft Access 2003 with VBA,” “Upgrader’s Guide to Microsoft Office System
2003,” “ICDL Exam Cram 2,” and “Absolute Beginner’s Guide to Microsoft Access 2003″ all by
Que. Currently, Susan volunteers as the Publications Director for Database Advisors at
http://www.databaseadvisors.com. You can reach her at ssharkins@gmail.com.
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6. How do I... Reseed a SQL Server identity column? | TechRepublic
10 Join the conversation! Add Your Opinion
Comments Follow via:
Staff Picks Top Rated Most Recent My Contacts See All Comments
IDENTITY columns 0
sqllion 20th Aug 2011 Votes
All features of SQL Server IDENTITY property of columns. And a handy procedure to
monitor all Identity columns. http://www.sqllion.com/2011/08/identity-in-sql/
View in thread
Useful, Thanks 0
epsilonv 24th Aug 2010 Votes
I had to execute the procedure twice for each table, first it set the current column and
next time it set the current identity. I did it through Visual Studio though so that might
have something to do... Read Whole Comment +
View in thread
RE: How do I... Reseed a SQL Server identity column? 0
sqllion Updated - 21st Aug 2011 Votes
For more information on Identifying and listing all the IDENTITY columns present in a
given database with filtering option available for specific tables, refer the below link:...
Read Whole Comment +
View in thread
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