Declare Foreign key in MySQL - mysql

Question

I have these database
=== Invoices ===
id
status
description
=== Invoice Items ===
id
invoice_id (FK)
item_name
description
To make this table I have made this MySQL command
CREATE TABLE IF NOT EXISTS `nt_invoices` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`status` varchar(45) NOT NULL DEFAULT '',
`description` text NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1 AUTO_INCREMENT=24 ;
CREATE TABLE IF NOT EXISTS `nt_invoice_items` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`invoice_id` int(11) NOT NULL,
`item_name` varchar(45) NOT NULL,
`description` text NOT NULL,
PRIMARY KEY (`id`),
KEY `invoice_id` (`invoice_id`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1 AUTO_INCREMENT=12 ;
My problem is that I want to declare a foreign key in the invoice_items table and to make the invoice_id the foreign key of invoices table id. So how to write that command? Any help and suggestions will be highly appreciated.

Answer 1

MyISAM does not support foreign keys. You need to use InnoDB (which is a better choice in all aspects anyway). Then it's just like in any other SQL dialect:
`invoice_id` int(11) NOT NULL references nt_invoices(id),
P.S. Also, always use utf8 encoding everywhere. It will bite you in the ass if you don't.

Answer 2

You should have innodb engine type for using foreign keys.
CREATE TABLE IF NOT EXISTS `nt_invoice_items` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`invoice_id` int(11) NOT NULL references nt_invoices(id)
`item_name` varchar(45) NOT NULL,
`description` text NOT NULL,
PRIMARY KEY (`id`),
KEY `invoice_id` (`invoice_id`)
) ENGINE=INNODB DEFAULT CHARSET=utf8;
OR if you want to use cascaded update delete:
CREATE TABLE IF NOT EXISTS `nt_invoice_items` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`invoice_id` int(11) NOT NULL,
`item_name` varchar(45) NOT NULL,
`description` text NOT NULL,
PRIMARY KEY (`id`),
KEY `invoice_id` (`invoice_id`),
FOREIGN KEY (invoice_id) REFERENCES nt_invoices(id)
ON DELETE CASCADE
ON UPDATE CASCADE,
) ENGINE=INNODB DEFAULT CHARSET=utf8;

Answer 3

You may also use ALTER command to declare FOREIGN KEY as follows:
Alter table table_name add foreign key(column_name)
references other_table_name(column);