This is a common interview question for SQL Server Database Developer. SQL Database Developer must prepare for this.

Sequence and identity, both used to generate an auto number, but the major difference is Identity is a table dependant and Sequence is independent of a table.

A Sequence Object introduced in SQL Server 2012 and Identity column introduced in SQL Server 6.0.

A Sequence is a User defined object and Identity column is a table property.

The same Sequence object can use with the multiple tables. The same Identity column cannot use with multiple tables, we can define identity column on each table individually.

If you define the same Sequence Object to multiple tables, it generates the sequence number database-wide and maintains a unique sequence number across the multiple tables. An Identity column generates a unique number at table only.

Sequence syntax (Start with 1 and Increment by 1):

CREATE SEQUENCE
  dbo.Sequence_Name AS INT
 START WITH 1
 INCREMENT BY 1
GO

Identity syntax (Start with 1 and Increment by 1):

CREATE TABLE dbo.Table_Name
( Id INT IDENTITY(1,1),
  Name VARCHAR(50) )
GO

Check the next value of Sequence object:

SELECT (NEXT VALUE FOR dbo.Sequence_Name) AS SeqNextValue

Check the next value of Identity Column:

SELECT SCOPE_IDENTITY()

(write after use of Identity column)

Check the current value of Sequence Object:

SELECT Current_Value 
FROM Sys.Sequences 
WHERE name='Sequence_Name'

Check the current value of Identity Column:

SELECT IDENT_CURRENT('Table_Name') 
 AS 'IdentityCurrentValue'

You can modify a Sequence object using ALTER Statement.

ALTER SEQUENCE dbo.Sequence_Name
RESTART WITH 100
INCREMENT BY 5
GO

You can only reset the value of an Identity column using DBCC CHECKIDENT().

DBCC CHECKIDENT('Table_Name', RESEED,100)

In Sequence New values can be generated in an UPDATE statement, but in Identity new value cannot be generated in an UPDATE statement.

You can define a maximum value for Sequence Object (Using MAXVALUE). You cannot provide a maximum value to Identity Column.

Once your Sequence object reached to MAXVALUE, you can also define a CYCLE option to restart it from the beginning. You cannot restart an Identity column value.

You can also enable data CACHE for a Sequence Object. You cannot enable data CACHE for an Identity Column and it uses table related data cache.

A User must require an access permission for Sequence Object. For Identity column, User requires only table related permissions.

Performance wise, Identity column is 3x faster than Sequence object.

In this post, I am sharing a script to find orphaned Sequence objects which not owned by any column of PostgreSQL Database.

When we define a column using SERIAL or BIGSERIAL, PostgreSQL internally creates one sequence for it and associate with it.
In RDBMS, A Sequence is an independent object, and you can use same sequence object for multiple tables as well.

In a development environment, we are executing DDL frequently commands like multiple CREATE or DROP statements, so there is possibility of Orphaned Sequence.

A Database Administrator can use this script to remove unwanted Sequence and make sure that Postgres database is up to date.

SELECT 
	ns.nspname AS SchemaName
	,c.relname AS SequenceName
FROM pg_class AS c
JOIN pg_namespace AS ns 
	ON c.relnamespace=ns.oid
WHERE c.relkind = 'S'
  AND NOT EXISTS (SELECT * FROM pg_depend WHERE objid=c.oid AND deptype='a')
ORDER BY c.relname;

SQL Server 2012 introduced a Sequence object with lots of functionality, but unfortunately, we can only create Sequence Object with Number data-types.

We can create a Sequence object for a list of below data-types:

tinyint
smallint
int
bigint
decimal
numeric

But in some situations, we also require alphanumeric Sequence Number, and for that, we should apply special FORMAT on the value of Sequence Number.

Below is a full demonstration to FORMAT a Sequence value and make it alphanumeric.

First, Create sample table:

CREATE TABLE dbo.tbl_TestVarcharSequence
(
	VarcharID VARCHAR(10)
	,Name VARCHAR(255)
	,CONSTRAINT pk_tbl_TestVarcharSequence_VarcharID PRIMARY KEY(VarcharID)
)
GO

Create a Sequence object:

CREATE SEQUENCE dbo.seq_TestVarcharSequenceNumber AS 
INT START WITH 1
INCREMENT BY 1;
GO

Create Sequence constraint using FORMAT function:

ALTER TABLE dbo.tbl_TestVarcharSequence
ADD CONSTRAINT seq_tbl_TestVarcharSequence_VarcharID DEFAULT 
FORMAT((NEXT VALUE FOR dbo.seq_TestVarcharSequenceNumber),'ABC000000#') 
FOR VarcharID;
GO

Insert few sample records:

INSERT INTO dbo.tbl_TestVarcharSequence (Name) 
VALUES ('Anvesh'),('Alex'),('Roy'),('Bony')
GO

The Result:

SELECT * FROM dbo.tbl_TestVarcharSequence

You can find Alphanumeric Sequence Number in the above result.