Sequence in SQL Server

Sequence in SQL Server

Introduction:
A sequence is an object in SQL Server that generates a series of numeric values in an ascending or descending order. It is commonly used to provide unique and sequential values for primary key columns. Sequences are helpful when you need to generate unique identifiers automatically and don't want to rely on identity columns or GUIDs.

Creating a Sequence:
To create a sequence in SQL Server, you can use the CREATE SEQUENCE statement. Below is the syntax:

sql
CREATE SEQUENCE schema_name.sequence_name
    [ AS { built_in_integer_type | user-defined_integer_type } ]
    START WITH start_value
    INCREMENT BY increment_value
    { MINVALUE min_value } | { NO MINVALUE }
    { MAXVALUE max_value } | { NO MAXVALUE }
    { CYCLE | { NO CYCLE } }
    { CACHE cache_size } | { NO CACHE };

Explanation of Parameters:

• schema_name: The name of the schema where the sequence will be created.
• sequence_name: The name of the sequence.
• built_in_integer_type or user-defined_integer_type: The data type of the sequence, such as INT, BIGINT, SMALLINT, etc.
• start_value: The first value generated by the sequence.
• increment_value: The value by which the sequence will be incremented for each new value.
• min_value: The minimum value the sequence can generate.
• max_value: The maximum value the sequence can generate.
• CYCLE: If specified, the sequence will restart from the MINVALUE after reaching MAXVALUE. If not specified, the sequence will throw an error after reaching MAXVALUE.
• CACHE: The number of sequence values that SQL Server pre-allocates for performance optimization.

Example:
Let's create a simple sequence that starts at 1 and increments by 1:

sql
CREATE SEQUENCE dbo.MySequence
    AS INT
    START WITH 1
    INCREMENT BY 1
    NO MINVALUE
    NO MAXVALUE
    NO CYCLE
    CACHE 20;

Using the Sequence:
You can use the sequence to generate values in various ways. One common use is to populate a primary key column automatically:

sql
CREATE TABLE dbo.MyTable (
    ID INT PRIMARY KEY DEFAULT NEXT VALUE FOR dbo.MySequence,
    Name VARCHAR(50)
);

INSERT INTO dbo.MyTable (Name) VALUES ('John');
INSERT INTO dbo.MyTable (Name) VALUES ('Jane');
INSERT INTO dbo.MyTable (Name) VALUES ('Mike');

The NEXT VALUE FOR function retrieves the next value from the sequence.

Modifying Sequence:
You can modify a sequence using the ALTER SEQUENCE statement. For example, to change the increment to 2:

sql
ALTER SEQUENCE dbo.MySequence
    INCREMENT BY 2;

Resetting Sequence:
To reset the sequence value, you can use the ALTER SEQUENCE statement with the RESTART option:

sql
ALTER SEQUENCE dbo.MySequence
    RESTART WITH 100;

This sets the next value generated by the sequence to 100.

Conclusion:
Sequences are a powerful feature in SQL Server that provide an easy way to generate unique and sequential numeric values. They are useful for generating primary key values, unique identifiers, or any situation where sequential numbers are needed.