Understanding and Working with Identity Columns in SQL Server

In SQL Server, an identity column is a special type of column that automatically generates a unique integer value for each new row inserted into a table. This can be useful for creating primary keys or other unique identifiers. However, working with identity columns can sometimes lead to confusion, especially when trying to insert explicit values into these columns.

By default, SQL Server does not allow you to insert explicit values into an identity column. If you try to do so, you will receive an error message stating that "Cannot insert explicit value for identity column in table ‘table’ when IDENTITY_INSERT is set to OFF." This is because the IDENTITY_INSERT option is set to OFF by default, which means that SQL Server will automatically generate values for identity columns.

To understand how to work with identity columns, let’s consider an example. Suppose we have a table called Operations with the following structure:

CREATE TABLE Operations (
    OperationID INT IDENTITY(1,1) PRIMARY KEY,
    OpDescription NVARCHAR(50),
    FilterID INT
);

In this example, the OperationID column is an identity column that automatically generates a unique integer value for each new row inserted into the table.

If we try to insert an explicit value into the OperationID column, like this:

INSERT INTO Operations (OperationID, OpDescription, FilterID)
VALUES (20, 'Hierarchy Update', 1);

We will receive the error message mentioned earlier. To resolve this issue, we have a few options.

One option is to set the IDENTITY_INSERT option to ON for the table, like this:

SET IDENTITY_INSERT Operations ON;

INSERT INTO Operations (OperationID, OpDescription, FilterID)
VALUES (20, 'Hierarchy Update', 1);

SET IDENTITY_INSERT Operations OFF;

This will allow us to insert explicit values into the OperationID column. However, it’s essential to note that setting IDENTITY_INSERT to ON can have unintended consequences, such as affecting other users who are trying to access the table.

Another option is to simply omit the OperationID column from the INSERT statement, like this:

INSERT INTO Operations (OpDescription, FilterID)
VALUES ('Hierarchy Update', 1);

This will allow SQL Server to automatically generate a unique value for the OperationID column.

It’s also worth noting that if you are using Entity Framework or other ORMs, you may need to update your entity model to reflect the identity column. For example:

[DatabaseGenerated(DatabaseGeneratedOption.Identity)]
public int OperationID { get; set; }

In conclusion, working with identity columns in SQL Server requires careful consideration of how to insert values into these columns. By understanding the IDENTITY_INSERT option and how to use it effectively, you can avoid common errors and ensure that your database is properly designed and maintained.