default or null are not allowed as explicit identity values

3 min read 01-10-2024

default or null are not allowed as explicit identity values

When working with databases, particularly with SQL Server, you might encounter the error message: "Default or Null are not allowed as explicit identity values." This article delves into this error, exploring its causes, implications, and potential solutions, while also offering additional context and practical examples to ensure you can navigate this challenge effectively.

What Are Identity Columns?

Identity columns are a feature in SQL databases that automatically generate unique values for new rows. This is particularly useful for primary key columns, where each row needs a unique identifier. The identity value is typically incremented automatically by the database.

For example, in SQL Server, you can define a column as an identity column with a statement like this:

CREATE TABLE Users (
    UserId INT IDENTITY(1,1) PRIMARY KEY,
    UserName NVARCHAR(100)
);

In this case, UserId will automatically increment for each new row added to the Users table.

The Error Explained

What Causes the Error?

The error "Default or Null are not allowed as explicit identity values" usually arises when:

Explicitly Providing a Null Value: If you attempt to insert a row with a NULL value for an identity column, you will trigger this error because the database expects a unique value.
Trying to Override the Identity Behavior: If you explicitly try to set the value of an identity column using an INSERT statement, such as:
```
INSERT INTO Users (UserId, UserName) VALUES (NULL, 'JohnDoe');
```
In this case, the attempt to assign NULL as an explicit identity value will lead to the mentioned error.

Example Scenario

Consider the following scenario:

You have the following table definition for a Products table:

CREATE TABLE Products (
    ProductId INT IDENTITY(1,1) PRIMARY KEY,
    ProductName NVARCHAR(100)
);

If you try to insert a product as follows:

INSERT INTO Products (ProductId, ProductName) VALUES (DEFAULT, 'Gadget');

You will get the error message indicating that DEFAULT or NULL are not permitted for ProductId because it's an identity column.

Solutions to Resolve the Error

Here are practical solutions to handle this error effectively:

1. Omit the Identity Column

The simplest way to avoid the error is to exclude the identity column from your INSERT statement. The database will automatically generate the identity value.

INSERT INTO Products (ProductName) VALUES ('Gadget');

2. Use SET IDENTITY_INSERT

If you need to insert specific values into an identity column for a particular situation (like data migration or seeding), you can enable identity inserts temporarily:

SET IDENTITY_INSERT Products ON;

INSERT INTO Products (ProductId, ProductName) VALUES (1, 'Gadget');

SET IDENTITY_INSERT Products OFF;

This allows you to explicitly set the identity values temporarily. Remember to turn it off after the operation to avoid further issues.

Additional Insights

Best Practices for Identity Columns

Avoid Explicit Assignments: It's best to let the database manage identity columns. Allowing it to automatically generate values ensures data integrity.
Use Appropriate Data Types: Ensure that your identity column's data type can accommodate the expected number of entries to avoid overflow errors.
Monitor Database Performance: Identity columns can affect performance if not managed properly. Regularly check for fragmentation and consider maintenance tasks.

Conclusion

Understanding how identity columns work and the implications of setting explicit identity values can help you avoid common pitfalls in SQL Server. The error "Default or Null are not allowed as explicit identity values" is straightforward to resolve with the appropriate knowledge and practices.

By applying these best practices and solutions, you will enhance your database management skills and ensure a more stable and reliable data architecture.

For more details, you can check the discussions and contributions on GitHub and various SQL Server community forums, where developers often share their experiences and solutions. Always ensure to give proper attribution to the original authors while discussing or implementing code from resources like GitHub.

By following the practices outlined in this article and being mindful of common errors, you'll be better equipped to handle identity columns in your SQL Server databases.