Mysql auto_increment conflicts with the Federated primary key

Before mysql 5.5, the auto_increment field must be the primary key. Sometimes, this auto-increment field does not have much practical significance, but we need multiple fields to form the primary key.

For example:

The user can only evaluate the purchased item once. The auto-increment ID of the evaluation is of no use. The Joint Primary Key composed of the user ID and the product ID is much more significant. If you want to retain the auto_increment field, you must use a later version of mysql.

Mysql> alter table 'order' drop primary key, add primary key ('User _ id', 'Order _ id ');
 
#1075-Incorrect table definition; there can be only one auto column and it must be defined as a key

The Federated primary key plays an important role in ensuring data uniqueness in the case of large traffic volumes.