In SQL Server, an identity column is a column that automatically generates a unique numeric value for new rows in a table. It’s commonly used for primary keys. Here’s how to manage identity columns in SQL Server:
1. Creating a Table with an Identity Column
To create a table with an identity column, you can use the IDENTITY property in the CREATE TABLE statement. The basic syntax includes specifying the seed (starting value) and increment (the amount by which the identity value is increased for each new row).
CREATE TABLE Employees (
EmployeeID INT IDENTITY(1,1) PRIMARY KEY,
FirstName NVARCHAR(50),
LastName NVARCHAR(50)
);
In the above example:
- IDENTITY(1,1) means the first value is 1 and each subsequent value will increase by 1.
2. Inserting Values into a Table with an Identity Column
When inserting data into a table with an identity column, you do not need to specify a value for the identity column; it will be generated automatically.
INSERT INTO Employees (FirstName, LastName)
VALUES ('John', 'Doe'),
('Jane', 'Smith');
3. Retrieving the Last Inserted Identity Value
To retrieve the last identity value generated by the session, you can use the SCOPE_IDENTITY() function immediately after the insert:
DECLARE @LastID INT;
INSERT INTO Employees (FirstName, LastName)
VALUES ('Alice', 'Johnson');
SET @LastID = SCOPE_IDENTITY();
SELECT @LastID AS LastInsertedID;
4. Managing Identity Values
a. Resetting the Identity Seed
If you need to reset the identity seed, you can use the DBCC CHECKIDENT command. For example:
DBCC CHECKIDENT ('Employees', RESEED, 1);
This will reset the next identity value to 1 (the next insert will use 2 if there are currently no rows).
b. Inserting Specific Values
If you need to insert a specific value into an identity column (not recommended in most scenarios), you can use the SET IDENTITY_INSERT property temporarily:
SET IDENTITY_INSERT Employees ON;
INSERT INTO Employees (EmployeeID, FirstName, LastName)
VALUES (10, 'Bob', 'Brown');
SET IDENTITY_INSERT Employees OFF;
5. Deleting Rows and Identity Values
If you delete rows, the identity values of those rows are not reused by default; new inserts will continue to use the next identity value. If you want to ensure there are no gaps in the identity values, you may consider reseeding as necessary.
6. Handling Identity Gaps
It's possible to encounter gaps in identity values, which can happen with deletions or rollbacks. Ensure that your application logic takes this into account if you need continuous sequences without gaps.
Summary
Managing identity columns in SQL Server involves defining them in table schemas, using them for automatic value generation, and performing various operations related to insertion and value retrieval. Always handle identity values carefully, especially when it comes to insertion and resetting the identity seed, to maintain data integrity in your applications.