Question

If I have two tables ‘users’ and ‘orders’ with the same user_id KEY, and I’d like to know if I update the user_id in one table, how do I get it to update in the other table automatically?

CREATE TABLE users(
id SMALLINT AUTO_INCREMENT,
user_id VARCHAR(15),
PRIMARY KEY(id),
KEY(user_id)
) ENGINE=MyISAM;

CREATE TABLE orders(
user_id VARCHAR(15),
//otherStuff,
KEY(user_id)
) ENGINE=MyISAM;

Answer 1

First, i recomend use InnoDB engine, not MyIsam.

InnoDB support foreign keys, in the create table stament include “ENGINE=InnoDB;”

Foreign keys enforce referential integrity between tables i.e. if you try to INSERT an order with a non-existent user id the system will throw an error.

The foreign key can be a reference case on change, ex, for change id in the reference table can be:

CREATE TABLE orders(
   user_id VARCHAR(15),
   //otherStuff,
   KEY(user_id),
   FOREIGN KEY (user_id) REFERENCES users(user_id)
     ON UPDATE CASCADE
) ENGINE=InnoDB;

This ON UPDATE CASCADE, change the value of user_id in case of change.