MS SQL Server - IDENTITY and SEQUENCE

Understanding IDENTITY and SEQUENCE in MS SQL Server

In MS SQL Server, IDENTITY and SEQUENCE are two features used to automatically generate unique values for columns, often for primary keys. This guide explores the differences between them and their applications, making it a valuable resource for both learning and job interviews.

IDENTITY Property

The IDENTITY property is used to generate unique numeric values for a column in an incremental manner. It’s commonly employed for auto-incrementing primary keys. Here’s an example of how to create a table with an IDENTITY column:


CREATE TABLE Employees
(
    EmployeeID INT IDENTITY(1,1) PRIMARY KEY,
    FirstName NVARCHAR(50),
    LastName NVARCHAR(50)
);

In this example, the EmployeeID column is set as an IDENTITY column, starting at 1 and incrementing by 1 for each new row inserted.

SEQUENCE Object

SEQUENCE is another method for generating unique values, and it provides more flexibility compared to IDENTITY. You can create a SEQUENCE object independently and use it with multiple tables. Here’s how to create a SEQUENCE object:


CREATE SEQUENCE OrderNumber
    AS INT
    START WITH 1
    INCREMENT BY 1;

Once the SEQUENCE object is created, you can use it with various tables:


CREATE TABLE Orders
(
    OrderID INT PRIMARY KEY,
    OrderNo INT DEFAULT (NEXT VALUE FOR OrderNumber),
    OrderDate DATETIME
);

In this example, the OrderNo column uses the SEQUENCE object OrderNumber to generate unique values.

Key Differences

While both IDENTITY and SEQUENCE serve the purpose of generating unique values, they have notable differences:

Flexibility: SEQUENCE is more versatile, as you can use a single SEQUENCE object with multiple tables.
Explicit Value Insert: With SEQUENCE, you can explicitly insert values generated by the SEQUENCE into a column, which is not possible with IDENTITY.
Range: SEQUENCE allows you to define the range of possible values, while IDENTITY operates within the INT data type limit.

Applications

IDENTITY is commonly used when you need a simple auto-incrementing primary key in a single table. It’s straightforward to set up and ideal for scenarios where you don’t require the additional features of SEQUENCE.

SEQUENCE, on the other hand, is beneficial when you need to generate unique values for multiple tables using a single SEQUENCE object. It’s suitable for cases where you want more control over the generated values and their range.

Best Practices

When using IDENTITY or SEQUENCE, it’s essential to follow best practices:

Data Type Selection: Choose an appropriate data type for the IDENTITY or SEQUENCE column based on your requirements.
Primary Key Selection: Ensure that the column using IDENTITY or SEQUENCE is suitable as a primary key.
Increment Value: Be mindful of the increment value to avoid exhausting the available values prematurely.

Conclusion

IDENTITY and SEQUENCE are valuable features in MS SQL Server for generating unique values, particularly for primary keys. Understanding the differences between these features, their applications, and best practices for their use can greatly benefit your SQL Server database design and development efforts. Whether you’re a beginner learning about database design or preparing for a job interview, this knowledge is essential in building robust and efficient database systems.

MS SQL Server – IDENTITY and SEQUENCE