Have you ever come across the CYCLE keyword in SQL Server SEQUENCE and wondered what it does? In this blog post, we will explore the concept of the CYCLE keyword and how it affects the behavior of a SEQUENCE in SQL Server.

Before we dive into the details, let’s quickly recap what a SEQUENCE is. A SEQUENCE is an object in SQL Server that generates a sequence of numeric values according to a specified pattern. It is often used to generate unique identifiers for tables or to create a series of numbers for various purposes.

Now, let’s take a look at an example to understand the CYCLE keyword in action:

USE TempDB
GO

-- Create sequence
CREATE SEQUENCE dbo.SequenceID AS DECIMAL(3, 0) START WITH 1 INCREMENT BY -1 MINVALUE 1 MAXVALUE 3 CYCLE NO CACHE;
GO

SELECT NEXT VALUE FOR dbo.SequenceID; -- Guess the number
SELECT NEXT VALUE FOR dbo.SequenceID; -- Guess the number

-- Clean up
DROP SEQUENCE dbo.SequenceID;
GO

In this example, we create a SEQUENCE called “SequenceID” with a starting value of 1, an increment value of -1, and a minimum value of 1. The maximum value is set to 3, and the CYCLE keyword is specified.

Now, let’s try to predict the output of the above code. Since the starting value is 1 and the increment value is -1, we might assume that the value will decrement from 1 to 0. However, the minimum value is set to 1, which means the value cannot further decrement. What will happen in this case?

The natural assumption would be that it should throw an error. But surprisingly, it doesn’t! Instead of throwing an error, the code will output the value 3.

So, why does this happen? The answer lies in the CYCLE keyword. When the SEQUENCE reaches the minimum value or the lower end of the range, the CYCLE keyword cycles the values from the other end of the range. In our example, since we have a negative incremental value, when the query reaches the minimum value of 1, it cycles back to the maximum value of 3.

If your business requirement is such that the sequence should throw an error when it reaches the maximum or minimum value, you should not use the CYCLE keyword. Without the CYCLE keyword, the SEQUENCE will behave as expected and throw an error when the range is exhausted.

I hope this blog post has helped you understand the concept of the CYCLE keyword in SQL Server SEQUENCE. If you have any interesting puzzles or questions related to SQL Server, feel free to share them with me. I would be happy to explore them and share them on this blog with due credit to you.