Foreign key reference tableA.column or tableB.column - mysql

Question

Is it possible to have a foreign key (InnoDB) reference two possible tables?
If not, is there a workaround for this?
There are two different subjects that contain the same field (interface_id). A third table references this field.
An example being:
physical_interface ( id, name, etc )
virtual_interface ( id, name, etc )
usage ( interface_id, etc )
I had an idea of using a view, but came across this related to SQL server: Can I have a foreign key referencing a column in a view in SQL Server? so it seems you cannot use views in foreign keys.
The alternative, I suppose, would be storing all intefaces in one table but I feel that would be less organized.

Answer 1

Well, the answer is interesting. MySQL 5.6.14, my version, won't stop you from creating a foreign key relationship with 2 tables but that is a bad idea. Let me show you why.
Create 2 master tables: test1 and test3
create table test1 (field1 int, primary key (field1));
create table test3 (field1 int, primary key (field1));
Create a dependent table that will have FK relationship with field1 of both tables.
create table test2 (
field0 int,
field1 int,
constraint fk_test2_test1
foreign key (field1)
references test1 (field1),
constraint fk_test2_test3
foreign key (field1)
references test3 (field1)
);
Great. Now let's add some data and see the issue:
insert into test1 values (1);
insert into test3 values (3);
Master tables are done. Let's add data to dependent table.
insert into test2 values (100, 1);
ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`test`.`test2`, CONSTRAINT `fk_test2_test3` FOREIGN KEY (`field1`) REFERENCES `test3` (`field1`))
insert into test2 values (100, 3);
ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`test`.`test2`, CONSTRAINT `fk_test2_test1` FOREIGN KEY (`field1`) REFERENCES `test1` (`field1`))
insert into test2 values (100, 2);
ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`test`.`test2`, CONSTRAINT `fk_test2_test1` FOREIGN KEY (`field1`) REFERENCES `test1` (`field1`))
So...don't do that.
Proposal
create table interface_type (
id int auto_increment,
type_name varchar(30),
primary key (id)
);
create table interface (
id int auto_increment,
interface_name varchar(30),
interface_type int,
primary key (id),
constraint fk_interface_type
foreign key (interface_type)
references interface_type (id)
);
create table `usage` (
id int auto_increment,
interface_id int,
primary key (id),
constraint fk_usage_interface
foreign key (interface_id)
references interface (id)
);
Now you can add as many types of interfaces in the interface_type table such as:
insert into interface_type (type_name) values ('physical'), ('virtual');
Then, add as many interfaces as you'd like in the interface table, and reference them in the usage table as needed.