Understanding Identity in SQL Server

Greetings, Dev! In this article, we will be discussing one of the most important concepts in SQL Server – Identity. Identity is a feature in SQL Server that allows users to automatically generate unique values for a specific column. This feature is commonly used to create primary keys in tables. Understanding how Identity works is crucial for anyone working with databases in SQL Server. Let’s dive into the details!

What is Identity?

In SQL Server, Identity is a feature that automatically generates unique values for a specific column in a table. The values can be integers, decimals, or even strings. The Identity feature is commonly used to create primary keys in tables. Every new row that is inserted in the table will get a unique Identity value assigned to it.

When a user creates a table with an Identity column, they can specify the starting value, the increment value, and the maximum and minimum values for the Identity column. SQL Server will then automatically generate values for the Identity column based on these specifications. This helps ensure that each value generated for the Identity column is unique.

Creating Tables with Identity Columns

To create a table with an Identity column, you need to use the IDENTITY keyword in the column definition. Here is an example:

Column Name	Data Type	Identity Specification
OrderID	int	IDENTITY(1,1)
CustomerID	nvarchar(50)
OrderDate	datetime

In the example above, the OrderID column is an Identity column. The IDENTITY(1,1) specification means that the starting value for the Identity column is 1, and the increment value is also 1. This means that every new row that is inserted in the table will get a unique value starting from 1.

Using Identity Columns in SQL Statements

When using Identity columns in SQL statements, you need to be careful to exclude the Identity column from any INSERT or UPDATE statements. This is because the Identity column is automatically generated by SQL Server and should not be manually inserted or updated.

Here is an example of an INSERT statement that excludes the Identity column:

INSERT INTO Orders (CustomerID, OrderDate) VALUES ('ALFKI', GETDATE())

In the example above, the OrderID column is excluded from the INSERT statement because it is an Identity column. SQL Server will automatically generate a unique value for the Identity column when the row is inserted.

FAQ

Q: Can I change the starting value of an Identity column?

A: Yes, you can change the starting value of an Identity column using the DBCC CHECKIDENT statement. Here is an example:

DBCC CHECKIDENT ('Orders', RESEED, 100)

In the example above, the starting value for the OrderID column in the Orders table is changed to 100.

Q: Can I make an existing column an Identity column?

A: No, you cannot make an existing column an Identity column. You will need to create a new table with an Identity column and copy the data from the old table to the new table.

Q: Can I have multiple Identity columns in a table?

A: No, you can only have one Identity column per table.

Q: What happens if I reach the maximum value for an Identity column?

A: When you reach the maximum value for an Identity column, SQL Server will return an error message and you will not be able to insert any more rows in the table. To avoid this, make sure to set the maximum value for the Identity column to a high enough value to accommodate all possible values.

Conclusion

Identity is a powerful feature in SQL Server that allows users to automatically generate unique values for a specific column in a table. It’s an important concept to understand for anyone working with databases in SQL Server. We hope that this article has been helpful in explaining the basics of Identity in SQL Server.