Sql Server Identity Columns Complete Guide

 Last Update:2025-06-23T00:00:00     .NET School AI Teacher - SELECT ANY TEXT TO EXPLANATION.    11 mins read      Difficulty-Level: beginner

Understanding the Core Concepts of SQL Server Identity Columns

SQL Server Identity Columns: Detailed Explanation and Important Information

Creating an Identity Column When creating a new table, you can define an identity column by using the IDENTITY property in the column definition. Here's a basic example:

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

In this example, the EmployeeID column is defined as an identity column with a seed value of 1 and an increment value of 1. This means that the first row inserted will receive EmployeeID of 1, the second row will receive 2 and so on.

Properties of Identity Columns

  • Seed: The starting value for the sequence.
  • Increment: How much the number increments each time a new row is inserted.
  • AUTO_INCREMENT: Unlike some database systems which have AUTO_INCREMENT keyword, in SQL Server we use the IDENTITY property to achieve the same effect.

Usage Scenarios Identity columns are commonly used when:

  • Creating tables that require simple unique identifiers.
  • Implementing primary keys without needing to manually assign identifier values.
  • Generating audit trail or logging mechanisms where row-level insertion timestamps are required.

System Tables and Views Related to Identity Columns To see if a column is set up as an identity column, you can query the system catalog views such as sys.columns and sys.tables.

SELECT 
    t.name AS TableName,
    c.name AS ColumnName,
    c.is_identity AS IsIdentity,
    c.seed_value AS SeedValue,
    c.increment_value AS IncrementValue
FROM 
    sys.tables t
INNER JOIN 
    sys.columns c ON t.object_id = c.object_id
WHERE 
    c.is_identity = 1;

The above query retrieves all columns in all tables that are declared as identity columns along with their seed and increment values.

Important Considerations

  1. Concurrency: SQL Server ensures that identity values are generated correctly even when concurrent inserts are occurring by reserving ranges.
  2. Deletion and Value Reuse: Deleting rows does not cause previously allocated identity values to be reused. SQL Server never reuses a deleted identity value.
  3. Transaction Rollback: If a transaction that involves inserting rows into an identity column is rolled back, any reserved identity values remain unused and do not reappear.
  4. Disable and Enable Identity Insert: You can manually insert values into an identity column by either setting SET IDENTITY_INSERT [tablename] ON before the inserts or by specifying column names explicitly in the insert statement.

Example:

-- To allow manual insertion
SET IDENTITY_INSERT Employees ON;

INSERT INTO Employees (EmployeeID, FirstName, LastName)
VALUES (500, 'John', 'Doe');

-- To stop allowing manual insertion
SET IDENTITY_INSERT Employees OFF;

Identity Values Overflow If the current identity value plus the increment exceeds the maximum value allowed for the data type of the identity column, an error will occur. To prevent this, ensure the seed and increment settings are appropriate for the expected volume of data.

Resetting Identity Values To reset the identity seed, you can use the DBCC CHECKIDENT command.

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

This resets the EmployeeID seed value to 0. The next inserted row will have a value of 1 assuming the default increment is 1.

Getting Last Generated Identity Value To retrieve the last identity value generated by an INSERT operation, use the SCOPE_IDENTITY(), @@IDENTITY, or OUTPUT clause.

Example:

INSERT INTO Employees (FirstName, LastName) VALUES ('Jane', 'Smith');
SELECT SCOPE_IDENTITY() AS LastGeneratedID;

Conclusion Identity columns provide a convenient way to generate sequential unique identifiers in SQL Server tables. They simplify data entry tasks by automating the assignment of unique IDs, reducing the risk of errors. However, there are specific properties and behaviors associated with these columns that require careful consideration during table design and data management to avoid issues related to concurrency, deletion, and overflow.

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Step-by-Step Guide: How to Implement SQL Server Identity Columns

Understanding Identity Columns

An Identity Column is a special column in SQL Server that automatically generates a unique numeric value for each row. They are often used as primary keys because the values are unique and auto-incremented.

Key Points:

  • Seed Value: The starting number for the identity column.
  • Increment Value: The difference between consecutive identity values.

Step-by-Step Examples

Step 1: Creating a Table with an Identity Column

Let's start by creating a table named Employees with an EmployeeID column that serves as the primary key and is an identity column.

SQL Code:

CREATE TABLE Employees (
    EmployeeID INT IDENTITY(1,1) PRIMARY KEY,
    FirstName NVARCHAR(50) NOT NULL,
    LastName NVARCHAR(50) NOT NULL,
    HireDate DATE NOT NULL
);

Explanation:

  • EmployeeID INT IDENTITY(1,1):
    • INT specifies the data type.
    • IDENTITY(1,1):
      • 1: Seed value. The first value inserted will be 1.
      • 1: Increment value. Each subsequent value will increase by 1.
  • PRIMARY KEY: Ensures that each EmployeeID is unique and not null.

Step 2: Inserting Data into the Table

Now, let's insert some data into the Employees table. Notice that you do not need to specify a value for the EmployeeID column because it will be automatically generated.

SQL Code:

INSERT INTO Employees (FirstName, LastName, HireDate) VALUES
('John', 'Doe', '2023-05-15'),
('Jane', 'Smith', '2023-06-20'),
('Alice', 'Johnson', '2023-07-25');

Explanation:

  • We only provide values for FirstName, LastName, and HireDate. The EmployeeID is generated automatically.

Verify the Insertion:

SELECT * FROM Employees;

Expected Output:

EmployeeID | FirstName | LastName | HireDate
-----------|-----------|----------|------------
1          | John      | Doe      | 2023-05-15
2          | Jane      | Smith    | 2023-06-20
3          | Alice     | Johnson  | 2023-07-25
  • EmployeeID 1: Automatically generated with a seed value of 1.
  • EmployeeID 2: Automatically generated by incrementing the previous value by 1.
  • EmployeeID 3: Similarly, it is 2 + 1.

Step 3: Modifying the Identity Property

Sometimes, you might need to change the seed or increment values of an existing identity column.

Example: Changing the Increment Value to 2

Let's change the increment value to 2, so future inserts will add 2 to the last generated EmployeeID.

  1. Disable the Identity Property Temporarily:
DBCC CHECKIDENT ('Employees', NORESEED);
ALTER TABLE Employees NOCHECK CONSTRAINT ALL;
  1. Drop the Existing Identity Column:
ALTER TABLE Employees DROP COLUMN EmployeeID;
  1. Add a New Identity Column with the New Increment Value:
ALTER TABLE Employees ADD EmployeeID INT IDENTITY(1,2) PRIMARY KEY;
  1. Insert New Rows to Verify the Change:
INSERT INTO Employees (FirstName, LastName, HireDate) VALUES
('Bob', 'Brown', '2023-08-30'),
('Charlie', 'Davis', '2023-09-05');
  1. Verify the New Identity Values:
SELECT * FROM Employees;

Expected Output:

EmployeeID | FirstName | LastName | HireDate
-----------|-----------|----------|------------
1          | John      | Doe      | 2023-05-15  -- Original
2          | Jane      | Smith    | 2023-06-20  -- Original
3          | Alice     | Johnson  | 2023-07-25  -- Original
5          | Bob       | Brown    | 2023-08-30  -- New, increment of 2
7          | Charlie   | Davis    | 2023-09-05  -- New, increment of 2
  • EmployeeID 1 to 3: Retained original values.
  • EmployeeID 5: 3 (last original value) + 2.
  • EmployeeID 7: 5 + 2.

Note: Changing the identity property can be complex and may cause data loss if not handled correctly. Always back up your data before making significant structural changes.

Step 4: Manually Overriding the Identity Value (Optional and Cautionary)

In some rare scenarios, you might need to insert a specific value into an identity column. However, this is generally discouraged because it can lead to conflicts and should be done with caution.

Disable Identity Insert Temporarily:

SET IDENTITY_INSERT Employees ON;

Insert a Specific Identity Value:

INSERT INTO Employees (EmployeeID, FirstName, LastName, HireDate) VALUES
(10, 'David', 'Miller', '2023-10-10');

Verify the Insertion:

SELECT * FROM Employees;

Expected Output:

EmployeeID | FirstName | LastName | HireDate
-----------|-----------|----------|------------
1          | John      | Doe      | 2023-05-15
2          | Jane      | Smith    | 2023-06-20
3          | Alice     | Johnson  | 2023-07-25
5          | Bob       | Brown    | 2023-08-30
7          | Charlie   | Davis    | 2023-09-05
10         | David     | Miller   | 2023-10-10  -- Manually inserted

Re-enable Identity Insert:

After inserting the specific value, remember to re-enable the identity insert:

SET IDENTITY_INSERT Employees OFF;

Important: Always set SET IDENTITY_INSERT to OFF after your manual insert to prevent future inserts from failing.

Step 5: Retrieving the Last Inserted Identity Value

When you insert a new row into a table with an identity column, you might need to retrieve the identity value of that row immediately. SQL Server provides several ways to do this.

Using SCOPE_IDENTITY():

SCOPE_IDENTITY() returns the last identity value inserted into any table in the current session and the current scope.

INSERT INTO Employees (FirstName, LastName, HireDate) VALUES
('Emily', 'Wilson', '2023-11-15');

SELECT SCOPE_IDENTITY() AS LastInsertedID;

Expected Output:

LastInsertedID
---------------
12
  • 12: The next identity value, following the last inserted value of 10 with an increment of 2.

Using IDENT_CURRENT('TableName'):

IDENT_CURRENT returns the last identity value generated for a specific table in any session and the current scope.

INSERT INTO Employees (FirstName, LastName, HireDate) VALUES
('Michael', 'Jones', '2023-12-20');

SELECT IDENT_CURRENT('Employees') AS LastInsertedID;

Expected Output:

LastInsertedID
---------------
14
  • 14: The next identity value, continuing the increment pattern.

Using OUTPUT Clause (SQL Server 2005 and later):

The OUTPUT clause allows you to return data from a DML statement (INSERT, UPDATE, DELETE) directly.

DECLARE @InsertedIDs TABLE (EmployeeID INT);

INSERT INTO Employees (FirstName, LastName, HireDate)
OUTPUT INSERTED.EmployeeID INTO @InsertedIDs
VALUES
('Sophia', 'Davis', '2024-01-15');

SELECT EmployeeID FROM @InsertedIDs;

Expected Output:

EmployeeID
-----------
16
  • 16: The new EmployeeID value for Sophia Davis.

Step 6: Re-seeding the Identity Column

Sometimes, you might need to reset or change the starting value of an identity column, especially after truncating a table or performing bulk data imports.

Re-seeding with DBCC CHECKIDENT:

Suppose you want to reset the seed value to 1:

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

Explanation:

  • RESEED, 0: Sets the next identity value to 1. The next inserted row will get EmployeeID 1.

Note: If the table has existing data, the next inserted row will use the reseeded value. Existing rows will not be affected.

Insert a New Row to Verify:

INSERT INTO Employees (FirstName, LastName, HireDate) VALUES
('William', 'Brown', '2024-02-20');

SELECT * FROM Employees;

Expected Output:

EmployeeID | FirstName | LastName | HireDate
-----------|-----------|----------|------------
1          | John      | Doe      | 2023-05-15
2          | Jane      | Smith    | 2023-06-20
3          | Alice     | Johnson  | 2023-07-25
5          | Bob       | Brown    | 2023-08-30
7          | Charlie   | Davis    | 2023-09-05
10         | David     | Miller   | 2023-10-10
12         | Emily     | Wilson   | 2023-11-15
14         | Michael   | Jones    | 2023-12-20
16         | Sophia    | Davis    | 2024-01-15
1          | William   | Brown    | 2024-02-20  -- New row with reseeded EmployeeID
  • EmployeeID 1: Reclaimed after reseeding. This could overwrite an existing row if not managed carefully.

Important: Re-setting the identity seed can lead to conflicts if there are existing rows with the same identity value. Ensure your table is empty or reseed to a value higher than the current maximum EmployeeID.

Step 7: Dropping an Identity Column

If you no longer need an identity column, you can drop it. However, this will also remove any auto-increment functionality.

Disable Identity Insert and Drop the Column:

SET IDENTITY_INSERT Employees OFF;

ALTER TABLE Employees DROP COLUMN EmployeeID;

-- Optionally, add a new column without the IDENTITY property
ALTER TABLE Employees ADD EmployeeID INT PRIMARY KEY;

Note: Dropping an identity column does not automatically transfer the auto-increment functionality to another column. You would need to implement similar logic manually.

Summary

  • Creating an Identity Column: Specify the IDENTITY(seed, increment) property during table creation.
  • Inserting Data: No need to specify a value for the identity column; it is auto-generated.
  • Modifying Identity Properties: Requires dropping and re-adding the column.
  • Retrieving Identity Values: Use functions like SCOPE_IDENTITY() or IDENT_CURRENT().
  • Re-seeding: Use DBCC CHECKIDENT to reset the seed value.
  • Dropping Identity Columns: Remove the column if no longer needed, understanding the implications.

Additional Resources

Top 10 Interview Questions & Answers on SQL Server Identity Columns

Top 10 Questions and Answers on SQL Server Identity Columns

1. What is an Identity Column in SQL Server?

2. How do you create an Identity Column in SQL Server?

Answer: To create an identity column, you specify the IDENTITY property when you define the column in the CREATE TABLE or ALTER TABLE statement. The syntax is IDENTITY(seed, increment). For example:

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

In this example, EmployeeID is an identity column starting at 1 and incrementing by 1 for each new row.

3. Can you insert a value into an Identity Column?

Answer: By default, you cannot explicitly insert a value into an identity column because SQL Server automatically generates these values. However, if you need to insert a specific value (typically for bulk inserts or when merging tables), you can set the IDENTITY_INSERT option to ON for the session or table. Remember to set it back to OFF afterward to maintain the identity property.

Example:

SET IDENTITY_INSERT Employees ON;
INSERT INTO Employees (EmployeeID, FirstName, LastName)
VALUES (3, 'John', 'Doe');
SET IDENTITY_INSERT Employees OFF;

4. How do you reset the Identity Column value?

Answer: To reset the identity column value, you can use the DBCC CHECKIDENT command. For example, to reset the next identity value to 1 and reseed:

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

This will set the next generated identity value to 1 for the next insert operation.

Caution: Use this sparingly and with caution as resetting can cause conflicts with existing keys, especially in production environments.

5. What happens if the identity column reaches its maximum value?

Answer: An identity column in SQL Server can store up to the maximum value of its data type:

  • INT: 2,147,483,647 (for identity(1,1))
  • BIGINT: 9,223,372,036,854,775,807

When the maximum value for the data type is reached, attempting to insert another row will result in the following error:

The int identity column in table 'YourTable' would overflow. Use the BIGINT identity column type to accommodate this data.

To mitigate this, use the BIGINT type for the identity column if you anticipate a large number of rows.

6. Can an identity column have a negative seed or increment?

Answer: Yes, an identity column can indeed have a negative seed and/or increment. For example:

CREATE TABLE TestTable (
    ID INT IDENTITY(-1,-1) PRIMARY KEY,
    Data VARCHAR(50)
);

However, using negative identity values and increments is uncommon and may cause confusion. Ensure that this design optimally fits your data requirements and that all business logic is compatible with negative keys.

7. How do you find the current identity value of a table without inserting a new row?

Answer: You can find the current identity value (the last value generated for the identity column) using SCOPE_IDENTITY(), @@IDENTITY, or IDENT_CURRENT(). However, to find the last generated value without inserting a new row, IDENT_CURRENT is the best option.

Example:

SELECT IDENT_CURRENT('Employees') AS CurrentIdentityValue;

8. Can an identity column be included in the WHERE clause?

Answer: Yes, an identity column can be used in the WHERE clause just like any other column. Identity columns are typically used to uniquely identify rows, making them ideal candidates for filtering data. Example:

SELECT *
FROM Employees
WHERE EmployeeID = 10;

9. Is an identity column affected by transactions?

Answer: Identity values generated within a transaction are permissible, and if the transaction is rolled back, the identity values are not reused. This behavior ensures that every identity value is unique, even if a transaction fails or is undone.

10. Can you have more than one identity column in a table?

Answer: No, a table in SQL Server can have only one identity column. The identity property is designed to ensure uniqueness for the primary key or similar unique identifiers, and having multiple identity columns would negate the primary purpose of this feature.

Understanding these aspects of identity columns in SQL Server can greatly enhance your ability to design efficient databases and manage data relationships effectively.

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