What does the identity attribute in SQL mean?

What is Identity in SQL, specific code examples are needed

In SQL, Identity is a special data type used to generate auto-incrementing numbers, it is commonly used To uniquely identify each row of data in the table. The Identity column is often used in conjunction with the primary key column to ensure that each record has a unique identifier. This article will detail how to use Identity and some practical code examples.

1. Basic usage of Identity

When creating a table, you can use the Identity attribute to specify a column as an auto-increment column. Typically, the Identity column is used with an integer type (such as INT, BIGINT, etc.). The following is an example of using Identity when creating a table:

CREATE TABLE [dbo].[Customers]  (
   [CustomerID] INT IDENTITY(1,1) PRIMARY KEY,
   [CustomerName] NVARCHAR(50) NOT NULL,
   [ContactName] NVARCHAR(50) NOT NULL,
   [Address] NVARCHAR(100) NULL
);

In the above code, the CustomerID column is designated as the Identity column. Each time data is inserted, it will automatically generate a unique auto-incrementing number as an identification .

2. Attributes of Identity

The Identity column has two important attributes: Seed (seed) and Increment (increment).

Seed specifies the starting value of the Identity column, which defaults to 1. In the above example, the Seed value is 1, which means that the CustomerID of the first row of data is 1, the CustomerID of the second row of data is 2, and so on.

Increment specifies the growth step of the Identity value, and the default is 1. In the above example, the Increment value is 1, which means that every time new data is inserted, the Identity value will automatically increase by 1. However, you can also change the growth step size by modifying the Increment value.

3. Inserting data using Identity

Inserting data using the Identity column is very simple, just do not include the value of the Identity column. When inserting data, the database automatically generates unique values ​​for the Identity column. The following example shows how to insert data into the Identity column:

INSERT INTO Customers (CustomerName, ContactName, Address)
VALUES ('ABC Company', 'John Smith', '123 Main Street');

INSERT INTO Customers (CustomerName, ContactName, Address)
VALUES ('XYZ Inc', 'Jane Doe', '456 Maple Avenue');

In the above code, we only inserted the values ​​of the CustomerName, ContactName and Address columns without specifying the value of CustomerID. The database automatically generates a unique value for CustomerID.

4. Get the value of Identity

Sometimes, we need to get the Identity value of the data we just inserted. In SQL Server, we can use the SCOPE_IDENTITY() function to obtain the Identity value of the most recently inserted data. The following code shows how to use the SCOPE_IDENTITY() function:

INSERT INTO Customers (CustomerName, ContactName, Address)
VALUES ('ABC Company', 'John Smith', '123 Main Street');

SELECT SCOPE_IDENTITY() AS LastID;

In the above code, we insert a new piece of data and use the SELECT statement to obtain the Identity value of the recently inserted data, which is stored in the LastID field middle.

5. Customize the Identity column

Sometimes, we may want to customize the starting value of the Identity column. SQL Server allows us to modify the current value of the Identity column through the DBCC CHECKIDENT statement. The following code shows how to use the DBCC CHECKIDENT statement:

DBCC CHECKIDENT ('Customers', RESEED, 100);

In the above code, we reset the current value of the Identity column of the Customers table to 100. The next time data is inserted, the Identity value will increment from 101.

Summary:

Identity is a commonly used auto-increment identifier in SQL, used to identify the uniqueness of each row of data in the table. When creating a table, you can use the Identity property to specify a column as the Identity column. When inserting data, if the value of the Identity column is not specified, the database will automatically generate a unique self-increasing number as an identifier. Through the SCOPE_IDENTITY() function, you can get the Identity value of the most recently inserted data. At the same time, you can use the DBCC CHECKIDENT statement to customize the current value of the Identity column. I hope this article will be helpful for you to study and use Identity in SQL.

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