MySQL在单个表上有多个外键约束

Question

I have 2 unrelated tables but each one has the same column type I called 'somefield'

CREATE TABLE `table1` (
 `id` int(10) NOT NULL AUTO_INCREMENT,
 `somefield` varchar(255) NOT NULL DEFAULT '',
 PRIMARY KEY (`id`),
 UNIQUE KEY `somefield` (`somefield`),
)

CREATE TABLE `table2` (
 `id` int(5) NOT NULL AUTO_INCREMENT,
 `somefield` int(20) DEFAULT NULL,
 PRIMARY KEY (`id`),
 UNIQUE KEY `somefield` (`somefield`)
)

After I insert into either of these tables, I insert into this table the 'somefield' value.

CREATE TABLE `table3` (
 `id` int(10) NOT NULL AUTO_INCREMENT,
 `somefield` varchar(255) NOT NULL DEFAULT '',
 PRIMARY KEY (`id`),
 UNIQUE KEY `somefield` (`somefield`),
 CONSTRAINT `FK_table3` FOREIGN KEY (`somefield`) REFERENCES `table1` (`somefield`) ON DELETE CASCADE ON UPDATE CASCADE
)

At the moment, if i delete a row in table1, the same row is deleted in table3. I can insert into table1 and do an insert on table3 without any problems but if i insert into table2 and try to insert into table3 i get this error

Integrity constraint violation: 1452 Cannot add or update a child row: a foreign key constraint fails

I know this is to do with the foreign key constraint in that it expects the 'somefield' value to be present in table1. What I want to know is how to add another constraint.

ALTER TABLE table3 ADD CONSTRAINT FK_table3 FOREIGN KEY (`somefield`) REFERENCES `table2` (`somefield`)  ON DELETE CASCADE ON UPDATE CASCADE;

Would i need to create 2 tables to act as a go between for table1/table3 and table2/table3?

Answer 1

I was able to sort this by creating a table which the other 3 tables all link to.

If a row is deleted in the new table, the resulting rows are deleted in the other tables.

CREATE TABLE IF NOT EXISTS `table1` (
  `id` tinyint(3) NOT NULL AUTO_INCREMENT,
  `somefield` varchar(12) DEFAULT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `somefield` (`somefield`)
);

CREATE TABLE IF NOT EXISTS `table2` (
  `id` tinyint(3) NOT NULL AUTO_INCREMENT,
  `somefield` varchar(12) DEFAULT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `somefield` (`somefield`)
);

CREATE TABLE IF NOT EXISTS `table3` (
  `id` tinyint(3) NOT NULL AUTO_INCREMENT,
  `somefield` varchar(12) DEFAULT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `somefield` (`somefield`)
);

CREATE TABLE IF NOT EXISTS `supertable` (
  `somefield` varchar(12) NOT NULL DEFAULT '',
  PRIMARY KEY (`somefield`),
  UNIQUE KEY `somefield` (`somefield`)
);

ALTER TABLE `table1`
  ADD CONSTRAINT `FK_table1` FOREIGN KEY (`somefield`) REFERENCES `supertable` (`somefield`) ON DELETE CASCADE ON UPDATE CASCADE;

ALTER TABLE `table2`
  ADD CONSTRAINT `FK_table2` FOREIGN KEY (`somefield`) REFERENCES `supertable` (`somefield`) ON DELETE CASCADE ON UPDATE CASCADE;

ALTER TABLE `table3`
  ADD CONSTRAINT `FK_table3` FOREIGN KEY (`somefield`) REFERENCES `supertable` (`somefield`) ON DELETE CASCADE ON UPDATE CASCADE;