System.Data.SqlClient.SqlException – Cannot insert explicit value for identity column

The error System.Data.SqlClient.SqlException – Cannot insert explicit value for identity column occurs when you try to explicitly insert a value into a column that is defined as an IDENTITY column in SQL Server. By default, SQL Server automatically generates values for IDENTITY columns, and explicit inserts are not allowed unless explicitly enabled.

What is an Identity Column?

An IDENTITY column is a column in a SQL Server table that automatically generates unique numeric values. It is commonly used for primary keys. The syntax for creating an IDENTITY column is:

CREATE TABLE ExampleTable (
    ID INT IDENTITY(1, 1) PRIMARY KEY, -- Starts at 1 and increments by 1
    Name NVARCHAR(50)
);

• IDENTITY(seed, increment): The seed is the starting value, and the increment is the value by which the column increases for each new row.

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Causes of the Error

1. Explicit Insert into Identity Column:

• You are trying to insert a value directly into an IDENTITY column.
• Example:
  sql INSERT INTO ExampleTable (ID, Name) VALUES (10, 'John'); -- Error

1. SET IDENTITY_INSERT OFF:

• By default, SET IDENTITY_INSERT is OFF, meaning explicit inserts into IDENTITY columns are not allowed.

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Solutions

1. Allow Explicit Inserts with SET IDENTITY_INSERT

If you need to insert explicit values into an IDENTITY column, you must enable IDENTITY_INSERT for the table.

Example:

   SET IDENTITY_INSERT ExampleTable ON;

   INSERT INTO ExampleTable (ID, Name) VALUES (10, 'John'); -- Allowed

   SET IDENTITY_INSERT ExampleTable OFF; -- Turn it off after the insert

Note:

• SET IDENTITY_INSERT can only be enabled for one table at a time in a session.
• You must specify the column list in the INSERT statement when IDENTITY_INSERT is enabled.

2. Do Not Insert into Identity Column

If you don’t need to insert explicit values, omit the IDENTITY column from the INSERT statement. SQL Server will automatically generate the value.

Example:

   INSERT INTO ExampleTable (Name) VALUES ('John'); -- ID is auto-generated

3. Reseed the Identity Column

If you want to reset the IDENTITY column to a specific value after inserting explicit values, use DBCC CHECKIDENT.

Example:

   DBCC CHECKIDENT ('ExampleTable', RESEED, 100); -- Reseeds the identity to 100

4. Check Table Design

If you don’t need the IDENTITY property for the column, you can remove it.

Example:

   ALTER TABLE ExampleTable DROP COLUMN ID;
   ALTER TABLE ExampleTable ADD ID INT PRIMARY KEY; -- Without IDENTITY

Warning: Dropping the IDENTITY property can have significant implications, so proceed with caution.

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Example Scenarios and Fixes

Scenario 1: Explicit Insert Without IDENTITY_INSERT

   INSERT INTO ExampleTable (ID, Name) VALUES (10, 'John'); -- Error

Fix:
Enable IDENTITY_INSERT:

   SET IDENTITY_INSERT ExampleTable ON;
   INSERT INTO ExampleTable (ID, Name) VALUES (10, 'John');
   SET IDENTITY_INSERT ExampleTable OFF;

Scenario 2: Auto-Generate Identity Value

   INSERT INTO ExampleTable (Name) VALUES ('John'); -- ID is auto-generated

Scenario 3: Reseed Identity Column

   DBCC CHECKIDENT ('ExampleTable', RESEED, 100);

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Best Practices

1. Avoid Explicit Inserts:

• Let SQL Server handle IDENTITY values unless you have a specific reason to insert explicit values.

1. Use SET IDENTITY_INSERT Sparingly:

• Only enable IDENTITY_INSERT when absolutely necessary, and disable it immediately after use.

1. Reseed with Caution:

• Reseeding the IDENTITY column can lead to duplicate values if not managed properly.

1. Use Identity for Primary Keys:

• IDENTITY columns are ideal for primary keys because they ensure uniqueness and simplify inserts.

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Debugging Steps

1. Check Table Definition:

• Verify if the column is an IDENTITY column.

   SELECT COLUMN_NAME, COLUMNPROPERTY(OBJECT_ID('ExampleTable'), COLUMN_NAME, 'IsIdentity') AS IsIdentity
   FROM INFORMATION_SCHEMA.COLUMNS
   WHERE TABLE_NAME = 'ExampleTable';

2. Review Insert Statements:

• Ensure you are not explicitly inserting into the IDENTITY column unless IDENTITY_INSERT is enabled.

2. Check for SET IDENTITY_INSERT:

• Ensure IDENTITY_INSERT is enabled if you need to insert explicit values.

By following these steps and best practices, you can resolve the Cannot insert explicit value for identity column error and manage IDENTITY columns effectively.