Understanding SQL Server Identity Column Error: "Explicit Value for Identity"

Understanding Identity Columns:

• In SQL Server tables, an identity column is a special type of column that automatically generates unique integer values for each new record inserted. This ensures that each record has a distinct identifier, which is often used as the primary key of the table.

The Error Message:

• This error arises when you attempt to insert data into a table that has an identity column, and you explicitly provide a value for that column in your INSERT statement.

Why It Occurs:

• SQL Server is designed to manage the identity column values itself. If you try to specify a value, it conflicts with this functionality.

Resolving the Issue:

There are two primary approaches to address this error:

1. Omit the Identity Column from the INSERT Statement:

   When you're inserting data, simply exclude the identity column from the list of columns you're specifying values for. SQL Server will automatically generate the appropriate value for the identity column. Here's an example:

   INSERT INTO MyTable (Column1, Column2, Column3)
   VALUES ('Value1', 'Value2', 'Value3');
   

2. Enable IDENTITY_INSERT and Include the Identity Column:

   • If you have a specific reason to control the identity column value (for example, when migrating data from another source), you can temporarily enable a setting called IDENTITY_INSERT for the table.
   • With IDENTITY_INSERT ON, you can then include the identity column in your INSERT statement and provide the desired value:

   SET IDENTITY_INSERT MyTable ON;
   
   INSERT INTO MyTable (IdentityColumn, Column1, Column2, Column3)
   VALUES (123, 'Value1', 'Value2', 'Value3');
   
   SET IDENTITY_INSERT MyTable OFF;  -- Important: Turn it off afterwards
   

   Important: Remember to turn off IDENTITY_INSERT after you're finished inserting the specific values, as you generally want SQL Server to manage identity column values automatically.

Approach 1: Omitting the Identity Column

This approach is ideal when you want SQL Server to automatically generate the identity value for each new record.

CREATE TABLE MyTable (
  ID int IDENTITY(1, 1) PRIMARY KEY,  -- Identity column with starting value 1 and increment 1
  Name varchar(50) NOT NULL,
  Age int
);

INSERT INTO MyTable (Name, Age)
VALUES ('Alice', 30), ('Bob', 25);

SELECT * FROM MyTable; -- Verify that ID values are automatically generated

In this example:

• We create a table MyTable with an identity column ID.
• We insert two records into the table, specifying values only for Name and Age.
• The ID column values will be automatically generated by SQL Server (likely starting from 1 in this case).

Approach 2: Enabling IDENTITY_INSERT

Use this approach only if you have a specific reason to control the identity value, such as data migration. Remember to turn IDENTITY_INSERT off afterward.

CREATE TABLE MyTable (
  ID int IDENTITY(1, 1) PRIMARY KEY,
  Name varchar(50) NOT NULL,
  Age int
);

SET IDENTITY_INSERT MyTable ON;  -- Enable IDENTITY_INSERT for the table

INSERT INTO MyTable (ID, Name, Age)
VALUES (100, 'Charlie', 40);  -- Specify a specific value for ID

SET IDENTITY_INSERT MyTable OFF;  -- Disable IDENTITY_INSERT after use

INSERT INTO MyTable (Name, Age)
VALUES ('David', 35);           -- ID will be automatically generated here

SELECT * FROM MyTable; -- Verify the inserted values

Here:

• We create the same MyTable with an identity column.
• We enable IDENTITY_INSERT temporarily.
• We insert a record with a specific ID value (100).
• We disable IDENTITY_INSERT.
• We insert another record, and the ID value will be automatically generated (likely 101).

1. Seeding Data with IDENTITY_INSERT in a Script:

   If you're dealing with a one-time data migration or initial data setup for your table, you can create a separate SQL script to insert data with IDENTITY_INSERT enabled. This script would handle the specific values you need to control, and then you'd keep IDENTITY_INSERT disabled in your regular application logic.

2. Separate Identity Table (Advanced):

   For more complex scenarios where you might need finer-grained control over identity values, you could explore creating a separate identity table. This table would have a single column (e.g., NextID with an identity) and a mechanism to retrieve the next available identity value. However, this approach requires more design and maintenance overhead.

3. Bulk Insert with IDENTITY_INSERT (Bulk Operations):

   If you're dealing with bulk data insertion, some SQL Server tools or techniques might allow you to specify IDENTITY_INSERT during the bulk operation itself. However, consult the specific tool or method documentation for details.

Remember, these approaches come with trade-offs in terms of complexity and maintainability. The first two approaches (omitting the identity column or using IDENTITY_INSERT in a dedicated script) are generally recommended for most cases.