SQL Error: Only One Auto Column Allowed, Must Be a Key

An auto column is a column that automatically generates a unique value for each new row inserted into a table. It's often used as a primary key to ensure each row has a unique identifier.

However, when defining a table in SQL, there can only be one auto column, and it must be defined as a key. This means the auto column must be declared as either the primary key or a unique key constraint.

For example, the correct way to define a table with an auto column as a primary key would be:

CREATE TABLE my_table (
  id INT AUTO_INCREMENT PRIMARY KEY,
  name VARCHAR(50),
  age INT
);

In this example, the 'id' column is defined as an auto column using the 'AUTO_INCREMENT' keyword and declared as the primary key using the 'PRIMARY KEY' keyword.

If you attempt to define multiple auto columns or an auto column without declaring it as a key, you'll receive an error message.