Question

Please consider the following SQL. I’m trying to create a table that references another. The foreign key isn’t being enforced and I don’t know why.

--drop database test;
create database test;
use test;

create table person (
    id         int         auto_increment,
    name         varchar(30),
    primary key (id)
);

create table message (
    id         int         auto_increment,
    senderid    int,
    receiverid    int,
    primary key (id),
    foreign key (senderid) references person(id),
    foreign key (receiverid) references person(id)
);

If I do a SHOW CREATE TABLE message, I get the following:

CREATE TABLE `message` (
 `id` int(11) NOT NULL AUTO_INCREMENT,
 `senderid` int(11) DEFAULT NULL,
 `receiverid` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `senderid` (`senderid`),
  KEY `receiverid` (`receiverid`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1 

Note that the foreign keys senderid/receiverid are just themselves. Without adding any rows into person:

mysql> insert into message (senderid, receiverid) values (1,2);
Query OK, 1 row affected (0.00 sec)

mysql> select * from person;
Empty set (0.00 sec)

mysql> select * from message;
+----+----------+------------+
| id | senderid | receiverid |
+----+----------+------------+
|  1 |        1 |          2 |
+----+----------+------------+
1 row in set (0.00 sec)

Why isn’t my foreign key working?

Answer 1

You are using MyISAM but, as documented under Foreign Key Differences:

For storage engines other than InnoDB, MySQL Server parses the FOREIGN KEY syntax in CREATE TABLE statements, but does not use or store it.

Suggest you use InnoDB instead.