The identity data type in SQL Server is a unique feature that allows for the auto-incrementing of a numeric value within a specified range. It is commonly used to generate primary key values for tables, ensuring that each record has a distinct identifier.
How does the identity data type work?
When a column is defined as an identity, SQL Server automatically assigns a new value to that column each time a new row is inserted into the table. The value starts at an initial seed and increments by a specified increment value. This process ensures that each inserted row receives a unique identifier.
Defining an identity column
To define an identity column in SQL Server, you need to specify the data type as “int” (for integer) or “bigint” (for larger numeric values), followed by the keyword “identity”. Here’s an example:
Example:
CREATE TABLE Customers (
CustomerID int IDENTITY(1,1),
FirstName varchar(50),
LastName varchar(50)
);
In this example, the CustomerID column is defined as an identity column. The seed value is set to 1, and the increment value is also set to 1. This means that when a new row is inserted into the Customers table, SQL Server will automatically generate and assign a unique CustomerID value.
Customizing the seed and increment values
By default, the seed value for an identity column is 1, and the increment value is also 1. However, you can customize these values according to your requirements.
To specify custom seed and increment values, you can modify the “IDENTITY” property of the column. For example:
Example:
CREATE TABLE Employees (
EmployeeID int IDENTITY(1000,5),
FirstName varchar(50),
LastName varchar(50)
);
In this example, the EmployeeID column starts with a seed value of 1000 and increments by 5 for each new row. This allows for larger gaps between generated values, which can be useful in certain scenarios.
Retrieving the generated identity value
After inserting a row into a table with an identity column, you may need to retrieve the generated identity value. You can use the “SCOPE_IDENTITY()” function to obtain the last inserted identity value within the current scope.
Example:
INSERT INTO Customers (FirstName, LastName)
VALUES ('John', 'Doe');
SELECT SCOPE_IDENTITY();
In this example, we insert a new customer into the Customers table and then use the “SCOPE_IDENTITY()” function to retrieve the generated CustomerID value for that customer.
Conclusion
The identity data type in SQL Server is a powerful feature that simplifies the generation of unique identifiers for records in tables. By automatically incrementing a numeric value, it ensures data integrity and simplifies database management.
Remember to customize seed and increment values when necessary and utilize functions like “SCOPE_IDENTITY()” to retrieve generated identity values. Incorporating these elements into your SQL Server database design can greatly enhance its efficiency and organization.