In SQL, an AUTO_INCREMENT field is a column attribute used primarily in MySQL, MariaDB, and some other relational database management systems. It automatically generates a unique value for each new row inserted into the table. This is particularly useful for creating primary keys.
Here's a basic example of how to define an AUTO_INCREMENT field in a CREATE TABLE statement in MySQL:
CREATE TABLE users (
user_id INT AUTO_INCREMENT PRIMARY KEY,
username VARCHAR(50),
email VARCHAR(100)
);
In this example, the user_id column is designated as AUTO_INCREMENT. When you insert a new row into the users table without specifying a value for user_id, the database automatically assigns a unique integer value to user_id. Each new row inserted will have a value for user_id that is one greater than the previous maximum value in the column.
In other database systems like PostgreSQL or SQL Server, the equivalent functionality is typically achieved using sequences or identity columns, respectively.