SQL Sequence Vs Identity Column

Let’s take a look at what a Sequence is in relation to an Identity Column in SQL Server. Did you know Sequence even existed? I didn’t until I was asked about them. It’s amazing how much you can skip over and never notice in SSMS. See this little folder, ever notice it under Programmability in Management Studio? Yep, it’s there: SQL Server has this very handy thing called Sequences. Sequences are a relatively new feature that have only existed since SQL Server 2012, but have long existed in Oracle (where there are no identity columns).

SQL Server

What is a Sequence?

Per MSDN, a sequence is a user-defined schema-bound object that generates a sequence of numeric values according to the specification with which the sequence was created. The sequence of numeric values is generated in an ascending or descending order at a defined interval and can be configured to restart (cycle) when exhausted. It's important to note the sequences can be cached and are not guaranteed to be in sequential order.

  1. CREATE SEQUENCE MySEQUENCE     START WITH 1      INCREMENT BY 1 ;  GO  

After creation, you can look at the properties in the GUI. Note you can set the increment by restarting the sequence, and even set min and max values.

SQL Server

SQL Server

How to Query

To see Sequence properties SELECT *FROM sys.sequences WHERE name='MySequence'  To Get Next Value SELECT NEXT VALUE FOR MySequence;

SQL Server

What’s an Identity Column?

A property of a table that is set by initial seed value (starting value). For each insert it assigns a new incremental value that is added to the identity value of the previous row that was loaded.

  1. CREATE TABLE MyIndentity (    NameID int IDENTITY(1,1),   FirstName varchar (25),   MiddleInt char(1),   LastName varchar(40)  );  

Note

After the field type IDENTITY, you declare the SEED (1), then INCREMENT Value (1). You can see this in the GUI below for the Column properties.

SQL Server

How to Query

Let's insert two records and see the NameID Identity column increment.

  1. INSERT MyIndentity  (FirstName, MiddleInt, LastName)  VALUES  ('Joe''K''Smith');  INSERT MyIndentity  (FirstName, MiddleInt, LastName)  VALUES  ('Jane''L''Doe');  SELECT * FROM MyIdentity  

SQL Server

Comparing the two
SQL Server

AttributeSequenceIdentity
Object LevelDatabaseTable
LimitCan set a limitLimited by data type INT vs BIG INT
ValuesGenerated by application call using NEXT VALUE FORGenerated on INSERT on a table
IncrementsDeclared as INCREMENT at setup and can be anything. Can be a negative number to cause the sequence to descend instead of ascending numbersDeclared as INCREMENT at setup and can be any positive number, numbers will ascend
ScopeGenerated outside the scope of a transactionGenerated within a transaction
Number AssignmentSequences can be preallocated (example assign me number 1-25)Cannot be preallocated, assigned in order by INSERT
GapsCan experience GapsCan experience Gaps
UniquenessNo, this number can be reset and reused.Often used as Primary Key (you must choose this property to ensure the unique value).

Summary

So, this was just a quick look what a Sequence is compared to an Identity column. Both can be very useful. If you're looking for a unique value your best bet is to go with an Identity Column and the Primary Key option. If you want just an auto-generated value to be able to use in an application outside of a table a Sequence is a sure bet. Play around with it, I am sure you can come up with a million and one uses for each.

***UPDATE NEW to SQL 2017 ***

Per MSDN there is a new option

IDENTITY_CACHE = { ON | OFF }

Applies to

SQL Server 2017 and Azure SQL Database (feature is in public preview)

Enables or disables identity cache at the database level. The default is ON. Identity caching is used to improve INSERT performance on tables with Identity columns. To avoid gaps in the values of the Identity column in cases where the server restarts unexpectedly or fails over to a secondary server, disable the IDENTITY_CACHE option. This option is similar to the existing SQL Server Trace Flag 272, except that it can be set at the database level rather than only at the server level.

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