SQL Server Identity Columns Step by step Implementation and Top 10 Questions and Answers

Last Update: April 01, 2025 18 mins read Difficulty-Level: beginner

SQL Server Identity Columns: A Comprehensive Guide

In the realm of database management, maintaining unique identifiers for rows in a table is crucial for ensuring data integrity and facilitating efficient data manipulation. SQL Server provides a feature known as "Identity Columns" to automatically generate sequential, unique numeric values for each row inserted into a table. This feature is particularly useful in scenarios where automatic numbering of rows is required, ensuring no manual intervention is needed for primary key assignment.

Understanding Identity Columns

An Identity Column is a specific type of column that automatically generates a unique value for each row inserted into a table. It is typically (but not exclusively) used for primary keys. The value of an Identity Column is generated based on two properties: the seed and the increment.

Seed: The seed is the initial value for the first row. By default, the seed value is 1.
Increment: The increment is the value added to the previous ID column value for each new row. The default increment is 1.

Identity Columns can be defined using the IDENTITY(seed, increment) property in a table definition. For example, the following SQL statement creates a table named Employees with an EmployeeID column as an Identity Column:

CREATE TABLE Employees (
    EmployeeID INT IDENTITY(1,1) PRIMARY KEY,
    Name NVARCHAR(100) NOT NULL,
    Position NVARCHAR(100) NOT NULL
);

In this example, EmployeeID is an Identity Column with a seed of 1 and an increment of 1, making it automatically assign unique, consecutive numbers to each new Employee row.

Generating Values Automatically

Whenever a new row is inserted into the table that contains an Identity Column, SQL Server automatically assigns a unique value to the Identity Column. This behavior is independent of the insertion order and ensures that no two rows have the same value in the Identity Column. For instance, if we insert three rows into the Employees table:

INSERT INTO Employees (Name, Position) VALUES ('John Doe', 'Developer');
INSERT INTO Employees (Name, Position) VALUES ('Jane Smith', 'Manager');
INSERT INTO Employees (Position, Name) VALUES ('Analyst', 'Sam Johnson');

SQL Server will automatically generate EmployeeID values for each row, resulting in something like:

| EmployeeID | Name | Position | |------------|--------------|----------| | 1 | John Doe | Developer| | 2 | Jane Smith | Manager | | 3 | Sam Johnson | Analyst |

Retrieving the Last Inserted Identity Value

There are often scenarios where you need to retrieve the last inserted identity value, such as referencing the newly added row in other tables or performing subsequent operations based on the generated ID.

SQL Server provides the SCOPE_IDENTITY(), @@IDENTITY, and OUTPUT clause functions for retrieving the last generated identity value:

SCOPE_IDENTITY(): Returns the last identity value generated in the current session and context.
@@IDENTITY: Returns the last identity value generated in the current session, regardless of the scope.
OUTPUT clause: Used in INSERT, MERGE, UPDATE, or DELETE statements to return a list of inserted, updated, or deleted rows.

Here's an example using SCOPE_IDENTITY():

DECLARE @LastInsertedID INT;

INSERT INTO Employees (Name, Position) VALUES ('Alice Brown', 'Trainee');
SET @LastInsertedID = SCOPE_IDENTITY();
SELECT @LastInsertedID AS LastInsertedID;

Managing Identity Columns

Once an Identity Column is created, its behavior can be managed through several operations:

Re-seeding: You can change the current identity value using the DBCC CHECKIDENT command. This is useful if you need to reset the identity value due to deleted rows or other reasons.

DBCC CHECKIDENT ('Employees', RESEED, 0); -- Resets the identity value to 0

Disabling and Re-enabling Identity Insert: Occasionally, you might need to manually insert values into an Identity Column. To do this, you must first allow manual inserts by setting IDENTITY_INSERT.

SET IDENTITY_INSERT Employees ON;
INSERT INTO Employees (EmployeeID, Name, Position) VALUES (5, 'David Lee', 'Intern');
SET IDENTITY_INSERT Employees OFF;

Best Practices for Using Identity Columns

Use Identity Columns for Primary Keys: Identity Columns are ideal for serving as the primary key in tables due to their unique and sequential nature.
Avoid Using Identity Columns for Business Keys: Identity Columns should not be used for business keys as they have no business meaning and can cause confusion.
Consider Data Type Appropriately: Choosing the appropriate data type for your Identity Column is crucial. For example, INT can hold up to 2,147,483,647 values, while BIGINT can handle up to 9,223,372,036,854,775,807 values.
Be Cautious with Re-seeding: Re-seeding Identity Columns can lead to duplicate key inserts if not managed carefully. Always ensure no existing identity values overlap with the new seed value.
Maintain Performance Considerations: Identity Columns do not pose significant performance overhead, but inserting large numbers of rows in a short period may cause locking and blocking issues. Proper indexing and partitioning can mitigate these problems.

Conclusion

SQL Server Identity Columns provide a powerful and convenient mechanism for automatically generating unique, sequential numeric identifiers for table rows. By leveraging Identity Columns effectively, database administrators and developers can streamline data management, ensure data integrity, and improve overall database performance. Understanding the intricacies of Identity Columns, including their properties, behavior, and best practices, is essential for making the most of this feature in SQL Server.

Examples, Set Route, and Run the Application: SQL Server Identity Columns for Beginners

Introduction

If you're starting out with SQL Server and databases, one of the fundamental concepts you'll encounter is Identity Columns. They are extremely useful for automatically generating unique sequential numbers, typically used as primary keys. The identity column simplifies the insertion process by eliminating the need to manually generate unique keys.

This guide will walk you through understanding, setting, and utilizing Identity Columns in SQL Server. We'll use practical, step-by-step examples to help you grasp how they work in a beginner-friendly manner.

Understanding Identity Columns

An identity column in SQL Server is a column that automatically generates a unique integer value for each new row in a table. The values start at a specified seed value and increment by a specified increment value.

Key Points:

Seed: The starting point for the identity values.
Increment: The amount by which the identity value increases for each new row.

Syntax to Define an Identity Column:

CREATE TABLE [tablename] (
    [columnname] INT IDENTITY(seed, increment),
    [other_columns]
)

Example Scenario

Let's assume we're developing a simple application for a bookstore that keeps track of books. Each book will have a unique identifier that we want to automate using an identity column.

Step-by-Step Guide

Setting Up SQL Server Environment
- Make sure you have SQL Server installed on your machine. If not, you can download SQL Server Express from the official Microsoft website.
- Open SQL Server Management Studio (SSMS) to connect to your SQL Server instance.
Create Database
- First, create a new database to work with.
```
CREATE DATABASE BookStoreDB;
GO
USE BookStoreDB;
```
Create a Table with Identity Column
- Next, create a new table named Books where we will use an identity column for the BookID.
```
CREATE TABLE Books (
    BookID INT IDENTITY(1,1) PRIMARY KEY,
    Title NVARCHAR(255) NOT NULL,
    Author NVARCHAR(255) NOT NULL,
    PublishedYear INT
);
```
- Explanation:
  - BookID INT IDENTITY(1,1) PRIMARY KEY means BookID is the primary key of the table and it will start from 1 and increment by 1 for each new row.

Insert Data into the Table

Now, let's insert some data into the Books table. Notice that you don't have to provide a value for the BookID as it's automatically generated.

INSERT INTO Books (Title, Author, PublishedYear)
VALUES ('SQL Server for Beginners', 'John Doe', 2022);
GO

INSERT INTO Books (Title, Author, PublishedYear)
VALUES ('Advanced SQL Techniques', 'Jane Smith', 2021);
GO

Query the Data
- Let's run a select query to see the data in the Books table.
```
SELECT * FROM Books;
```
- Expected Output:
  - BookID: 1, Title: SQL Server for Beginners, Author: John Doe, PublishedYear: 2022
  - BookID: 2, Title: Advanced SQL Techniques, Author: Jane Smith, PublishedYear: 2021

Handling Identity Columns in Application Logic

Suppose you're building an application in C# that interacts with this SQL Server database. Here’s how you might handle identity columns in the application logic.

C# Example:

using System;
using System.Data.SqlClient;

class Program
{
    static void Main(string[] args)
    {
        string connectionString = "server=localhost;database=BookStoreDB;trusted_connection=yes;";

        using (SqlConnection connection = new SqlConnection(connectionString))
        {
            connection.Open();

            SqlCommand command = new SqlCommand(
                "INSERT INTO Books (Title, Author, PublishedYear) OUTPUT INSERTED.BookID VALUES (@Title, @Author, @PublishedYear)", connection);

            command.Parameters.AddWithValue("@Title", "Learning C#");
            command.Parameters.AddWithValue("@Author", "Alice Johnson");
            command.Parameters.AddWithValue("@PublishedYear", 2023);

            int newBookID = (int)command.ExecuteScalar();

            Console.WriteLine("New Book ID: " + newBookID);
        }
    }
}

Explanation:
- We use the OUTPUT INSERTED.BookID clause in the SQL query to get the identity value of the newly inserted row.
- In the application, we execute the command and retrieve the new BookID.

Data Flow and Application Integration
- With the above setup, here’s how data flows through your application:
  - The application sends an insert request to the SQL Server database.
  - SQL Server processes the request and inserts the data into the Books table.
  - SQL Server generates a unique BookID and returns it to the application.
  - The application captures this BookID and can use it for further processing or display.

Best Practices

Choosing Seed and Increment Wisely: Choose seed and increment based on your use case and expected growth of your table.
Don’t Use Identity for Primary Keys if Not Required: While identity columns are useful for primary keys, avoid using them in situations where other columns can serve as unique identifiers more meaningfully.
Avoid Gaps in Values: Identity columns are guaranteed to be unique but not necessarily contiguous.
Use OUTPUT Clause: When inserting data, use the OUTPUT clause to get the newly generated identity value directly in your application logic.

Conclusion

Identity columns in SQL Server are a powerful feature for automatically generating unique sequential numbers, especially useful for primary keys. By following the steps provided in this guide, you should now have a solid understanding of:

How to create tables with identity columns.
How to insert data into tables with identity columns using both SQL and application logic.
How to retrieve and utilize identity values in your application.

With practice, these concepts will become second nature and you'll find them indispensable for managing unique identifiers in your databases effectively. Happy Coding!

Top 10 Questions and Answers on SQL Server Identity Columns

SQL Server identity columns are one of the essential features that allow automatic generation of unique values for a table's key columns. Identity columns simplify key management within a database, promoting data integrity and consistency. Here are ten frequently asked questions about SQL Server identity columns, along with their answers:

1. What are Identity Columns in SQL Server?

Answer: In SQL Server, an identity column is a column where the value is automatically generated when a new row is inserted into the table. This is typically used as a primary key or unique identifier, ensuring that each row in the table has a unique value. The identity property is defined with two parameters: Seed and Increment. Seed is the starting point of the identity value, while Increment defines the step size between consecutive identity values.

2. How Do You Create an Identity Column in SQL Server?

Answer: You can create an identity column when creating a new table by using the IDENTITY keyword. Here's an example:

CREATE TABLE Customers (
    CustomerID INT IDENTITY(1,1) PRIMARY KEY,
    FirstName NVARCHAR(50),
    LastName NVARCHAR(50),
    Email NVARCHAR(100)
);

In this example, CustomerID is the identity column. It starts at 1 (Seed = 1), and each new row will increment by 1 (Increment = 1).

3. Can You Change the Identity Property of an Existing Column in a Table?

Answer: Yes, you can change the identity property of an existing column using the ALTER TABLE statement along with ALTER COLUMN and SET IDENTITY_INSERT. However, changing the seed or increment of an existing identity column is not directly possible through SQL Server Management Studio or T-SQL commands. Instead, you must drop the column and recreate it with the new identity settings. Here's an example to drop and recreate the column:

ALTER TABLE Customers DROP COLUMN CustomerID;
ALTER TABLE Customers ADD CustomerID INT IDENTITY(10,2) PRIMARY KEY;

4. How Do Identity Columns Handle Gaps in Values?

Answer: SQL Server identity columns can have gaps in values due to various reasons such as transactions that are rolled back, rows that are deleted, or system-generated transactions. These gaps are normal and do not affect the functionality of the identity column. The next inserted row will take the next valid number in the sequence.

5. Can You Insert Values into an Identity Column?

Answer: By default, you cannot insert explicitly into an identity column. This behavior is controlled by the IDENTITY_INSERT setting, which is OFF by default. If you need to insert values explicitly, you must set IDENTITY_INSERT to ON for that session or table.

SET IDENTITY_INSERT Customers ON;
INSERT INTO Customers (CustomerID, FirstName, LastName, Email)
VALUES (5, 'John', 'Doe', 'john.doe@example.com');
SET IDENTITY_INSERT Customers OFF;

Remember to turn IDENTITY_INSERT OFF after the operation to prevent accidental data corruption.

6. What Happens When You Delete a Row with an Identity Column?

Answer: When you delete a row from a table with an identity column, the identity value for that row is freed up. Subsequent inserts into the table will not reuse the freed identity value; instead, they will continue with the next sequential value. This is another reason for the gaps that can appear in the identity column values.

7. How Do You Find the Current Identity Value for an Identity Column?

Answer: You can find the current identity value of an identity column using the SCOPE_IDENTITY(), IDENT_CURRENT(), or @@IDENTITY functions.

SCOPE_IDENTITY() returns the last identity value created in the current session and the current scope.
IDENT_CURRENT() returns the last identity value generated for a specific table in any session and any scope.
@@IDENTITY returns the last identity value created in the current session, which might include identity values created in triggers.

Here’s how you might use these functions:

INSERT INTO Customers (FirstName, LastName, Email)
VALUES ('Jane', 'Smith', 'jane.smith@example.com');

SELECT SCOPE_IDENTITY();
SELECT IDENT_CURRENT('Customers');
SELECT @@IDENTITY;

8. Can You Reset the Identity Value in SQL Server?

Answer: Yes, you can reset the identity value to a new seed value using the DBCC CHECKIDENT() command. This command can also be used to reseed the identity column and specify the NOCHECK option to ignore the current value in the column.

DBCC CHECKIDENT ('Customers', RESEED, 0);

This command will reset the identity seed of the Customers table to 0. The next row inserted will have an identity value of 1.

9. What Are the Benefits of Using Identity Columns?

Answer: The primary benefit of using identity columns is the automation and guarantee of unique, sequential values. They simplify primary key management, reduce errors, and eliminate the need for manually assigning unique keys.

Uniqueness: Identity columns ensure that no two rows have the same primary key value.
Ease of Use: They simplify inserting data into tables without having to worry about providing unique key values.
Performance Optimization: Identity columns can improve performance because the database engine can quickly identify and generate unique values.

10. Are There Any Considerations When Using Identity Columns?

Answer: While identity columns are powerful, they come with certain considerations:

Gaps: Identity columns do not guarantee consecutive numbers due to reasons such as transactions being rolled back or rows being deleted.
Performance: Although performance is usually efficient, frequent reseeding or resetting identity values can impact the performance of indexes and queries.
Portability: Identity columns are specific to SQL Server and might not be compatible with other database systems if you plan to port your database.
Security: Be cautious when enabling IDENTITY_INSERT to avoid accidental data corruption, as it allows explicit values to be inserted into the identity column.

In conclusion, SQL Server identity columns provide a robust and efficient way to manage unique identifiers within your database tables. Understanding how to create, manage, and use identity columns can significantly enhance your ability to work effectively with SQL Server databases.