Question

Many people had this problem already, but there was no fitting solution in other posts.

I have two tables, one named “sales”, the other named “host_flags”. I would like to have a foreign key for host_flags.sales_id to sales.id, but mysql won’t let me! I have primary indexes defined in each table, so I wonder why…

The host_flags table already has a foreign key on the column host_id, but even when I tried and created the foreign key for the sales id first, it wouldn’t let me.

The tables look like:

CREATE TABLE `sales` (
  `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `email` varchar(255) DEFAULT NULL,
  `password` varchar(255) DEFAULT NULL,
  `creation` datetime DEFAULT NULL,
  `lastupdate` timestamp NULL DEFAULT NULL ON UPDATE CURRENT_TIMESTAMP,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=latin1;

CREATE TABLE `host_flags` (
  `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `host_id` int(11) DEFAULT NULL,
  `sales_id` int(11) DEFAULT NULL,
  `creation` datetime DEFAULT NULL,
  `lastupdate` timestamp NULL DEFAULT NULL ON UPDATE CURRENT_TIMESTAMP,
  PRIMARY KEY (`id`),
  KEY `host_id6` (`host_id`),
  CONSTRAINT `host_id6` FOREIGN KEY (`host_id`) REFERENCES `hosts` (`id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

CREATE TABLE `hosts` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `creation` datetime NOT NULL,
  `lastupdate` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=32225 DEFAULT CHARSET=utf8 COLLATE=utf8_bin;

I get this error message:

MySQL said: Can't create table 'primarydata.#sql-191_1' (errno: 150)

Thanks!

Charles

SOLUTION FOUND

All ints of the primary indexes have to be either signed or unsigned – not mixed.

Answer 1

Typically:

I like to declare the FK constraints outside of the table definition after all tables have been constructed.

ALTER TABLE `tbl`
    ADD CONSTRAINT `constr`
        FOREIGN KEY `fk_id` REFERENCES `ftbl`(`id`)
            ON UPDATE CASCADE
            ON DELETE CASCADE;

This way I can make sure the problem isn’t something like the datatype of tbl.fk_id not being the same as the one of ftbl.id (including UNSIGNED as @Devart said). Or not having declared ftbl.id as unique. Regardless of the order of declaration of the tables.

After i do this i can integrate the constraint back into the table definition and take into account the order in which the tables need to be created to allow the constraint to be added.

You problem:

-- creating the sales table
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,

-- creating the host_flags table
`sales_id` int(11) DEFAULT NULL,

-- the sales.id is declared as unsigned
-- the host_flags.sales_id is declared signed