MySQL "Foreign key constraint is incorrectly formed" - mysql

Question

I have the follwing SQL commands:
CREATE TABLE IF NOT EXISTS `users`
(
`id` INTEGER NOT NULL auto_increment,
`username` VARCHAR(255) NOT NULL UNIQUE,
`password` VARCHAR(255) NOT NULL,
`root` TINYINT(1) DEFAULT 0,
`createdat` DATETIME NOT NULL,
`updatedat` DATETIME NOT NULL,
UNIQUE `username_unique` (`username`),
PRIMARY KEY (`id`)
)
engine=innodb;
CREATE TABLE IF NOT EXISTS `domains`
(
`id` INTEGER NOT NULL auto_increment,
`domain` VARCHAR(255) NOT NULL UNIQUE,
`createdat` DATETIME NOT NULL,
`updatedat` DATETIME NOT NULL,
`userid` INTEGER,
UNIQUE `domain_unique` (`domain`),
PRIMARY KEY (`id`),
FOREIGN KEY (`userid`) REFERENCES `users` (`id`) ON DELETE SET NULL ON
UPDATE CASCADE
)
engine=innodb;
CREATE TABLE IF NOT EXISTS `aliases`
(
`id` INTEGER NOT NULL auto_increment,
`source_username` VARCHAR(255) NOT NULL,
`source_domain` VARCHAR(255) NOT NULL,
`destination_username` VARCHAR(255) NOT NULL,
`destination_domain` VARCHAR(255) NOT NULL,
`enabled` TINYINT(1) NOT NULL DEFAULT 1,
`createdat` DATETIME NOT NULL,
`updatedat` DATETIME NOT NULL,
PRIMARY KEY (`id`),
FOREIGN KEY (`source_domain`) REFERENCES `domains` (`domain`)
)
engine=innodb;
CREATE TABLE IF NOT EXISTS `accounts`
(
`id` INTEGER NOT NULL auto_increment,
`username` VARCHAR(255) NOT NULL,
`domain` VARCHAR(255) NOT NULL,
`password` VARCHAR(255) NOT NULL,
`quota` INTEGER NOT NULL DEFAULT 500,
`enabled` TINYINT(1) NOT NULL DEFAULT 1,
`sendonly` TINYINT(1) NOT NULL DEFAULT 0,
`createdat` DATETIME NOT NULL,
`updatedat` DATETIME NOT NULL,
PRIMARY KEY (`id`),
FOREIGN KEY (`domain`) REFERENCES `domains` (`domain`)
)
engine=innodb;
if i try to run these i get the message:
"Foreign key constraint is incorrectly formed"
for the tables Alias and Accounts.
Its not possible to use primary keys in alias and accounts
Im using MariaDB 10.2
I hope somebody can tell me what is wrong with these statements.

Answer 1

You need to add a key/index to the domain field of the domains table, in order for it to be used as Foreign Key by another table.
If the table is already created and you want to add an index, use this.
CREATE INDEX domain ON domains(domain);
OR add the index while creating the table -
CREATE TABLE IF NOT EXISTS `domains`
(
`id` INTEGER NOT NULL auto_increment,
`domain` VARCHAR(255) NOT NULL UNIQUE,
`createdat` DATETIME NOT NULL,
`updatedat` DATETIME NOT NULL,
`userid` INTEGER,
UNIQUE `domain_unique` (`domain`),
PRIMARY KEY (`id`),
INDEX domain (domain),
FOREIGN KEY (`userid`) REFERENCES `users` (`id`) ON DELETE SET NULL ON
UPDATE CASCADE
)
engine=innodb;
For further reading (official documentation) regarding Foreign Key Contraint Errors on MariaDB this URL

Answer 2

You define an integer primary key. Use it:
CREATE TABLE IF NOT EXISTS `aliases`
(
`id` INTEGER NOT NULL auto_increment,
`source_username` VARCHAR(255) NOT NULL,
`source_domain_id` INTEGER NOT NULL,
`destination_username` VARCHAR(255) NOT NULL,
`destination_domain` VARCHAR(255) NOT NULL,
`enabled` TINYINT(1) NOT NULL DEFAULT 1,
`createdat` DATETIME NOT NULL,
`updatedat` DATETIME NOT NULL,
PRIMARY KEY (`id`),
FOREIGN KEY (`source_domain_id`) REFERENCES `domains` (`id`)
)
engine=innodb;