Sequences in MSSQL

In MSSQL Server, sequences are objects used to generate numeric values sequentially. They are often used to generate surrogate keys for tables. Here's how you can create and use a sequence in MSSQL:-

1. Create Sequence:

   

   CREATE SEQUENCE MySequence

       START WITH 1

       INCREMENT BY 1

       MINVALUE 1

       MAXVALUE 1000;

This creates a sequence named MySequence that starts at 1 and increments by 1, with values ranging from 1 to 1000.

2. Next Value:

   

   To retrieve the next value from the sequence, you can use the NEXT VALUE FOR function:

   SELECT NEXT VALUE FOR MySequence;

This will return the next value from the sequence without incrementing it.

3. Current Value:

   

If you want to see the current value of the sequence without advancing it, you can use the CURRENT_VALUE function:

   SELECT CURRENT_VALUE FOR MySequence;

   This will return the current value of the sequence.

4. Alter Sequence:

   

   You can alter a sequence to change its properties, such as increment value or maximum value:

   ALTER SEQUENCE MySequence

       INCREMENT BY 2;

This will change the increment value of the sequence to 2.

5. Drop Sequence:

   

   To remove a sequence from the database, you can use the DROP SEQUENCE statement:

   DROP SEQUENCE MySequence;

   This will delete the MySequence sequence from the database.

Sequences provide a convenient way to generate unique numeric values, especially for primary key columns in tables.

Here's an example of creating a sequence, retrieving values from it, and altering it with sample outputs:-

1. Create Sequence:

CREATE SEQUENCE MySequence

    START WITH 1

    INCREMENT BY 1

    MINVALUE 1

    MAXVALUE 1000;

Output:

Command(s) completed successfully.

2. Retrieve Next Value:

SELECT NEXT VALUE FOR MySequence AS NextValue;

Output:

NextValue

---------

1

3. Retrieve Current Value:

SELECT CURRENT_VALUE FOR MySequence AS CurrentValue;

Output:

CurrentValue

------------

1

4. Alter Sequence (Change Increment):

ALTER SEQUENCE MySequence

    INCREMENT BY 2;

Output:

Command(s) completed successfully.

5. Retrieve Next Value Again:

SELECT NEXT VALUE FOR MySequence AS NextValue;

Output:

NextValue

---------

3

6. Drop Sequence:

DROP SEQUENCE MySequence;

Output:

Command(s) completed successfully.

This demonstrates the creation of a sequence, retrieving its values, altering it, and finally dropping it from the database.

Below are five frequently asked questions about sequences in Microsoft SQL Server:-

1. What is a sequence in SQL Server?

   - A sequence is a database object in SQL Server that generates a sequence of numeric values according to specified properties such as starting value, increment, minimum value, and maximum value. It is often used to generate unique numeric identifiers automatically, such as surrogate keys for tables.

2. How do I create a sequence in SQL Server?

   - To create a sequence in SQL Server, you can use the CREATE SEQUENCE statement followed by the sequence name and its properties such as START WITH, INCREMENT BY, MINVALUE, and MAXVALUE. For example:

     CREATE SEQUENCE MySequence

         START WITH 1

         INCREMENT BY 1

         MINVALUE 1

         MAXVALUE 1000;

3. How do I retrieve the next value from a sequence in SQL Server?

   - You can retrieve the next value from a sequence in SQL Server using the NEXT VALUE FOR function. For example:

     SELECT NEXT VALUE FOR MySequence AS NextValue;

4. Can I reset a sequence in SQL Server?

   - Yes, you can reset a sequence in SQL Server by altering its properties using the ALTER SEQUENCE statement. You can set the RESTART WITH property to the desired value to reset the sequence. For example:

    

     ALTER SEQUENCE MySequence

         RESTART WITH 1;

   

5. Are sequences guaranteed to be unique in SQL Server?

   - Yes, sequences in SQL Server are designed to guarantee uniqueness. Each value generated by a sequence is unique within the scope of the sequence object. However, it's essential to ensure that the sequence is used appropriately within the context of your database schema to maintain data integrity.