Foreign key on different tables - mysql

Question

I have a images table:
Images
image_id
image_type ('foo', 'bar')
type_id
How can I create a foreign key so if the image_type is foo, type_id should relate to foo table, if image_type is bar to bar table.

Answer 1

What you're trying to do is to create a supertype. In SQL this is best modeled like below (IMO at least). My apologies if the syntax isn't quite right for MySQL. I usually use MS SQL Server.
CREATE TABLE Image_Types (
image_type_id INT NOT NULL,
image_type CHAR(3) NOT NULL CONSTRAINT CHK_Image_Types_image_type CHECK (image_type IN ('Foo', 'Bar')),
CONSTRAINT PK_Image_Types PRIMARY KEY (image_type_id, image_type)
)
CREATE TABLE Foo (
image_type_id INT NOT NULL,
image_type CHAR(3) NOT NULL CONSTRAINT CHK_Foo_image_type CHECK (image_type IN ('Foo')),
some_foo_specific_column VARCHAR(100) NOT NULL,
CONSTRAINT PK_Foo PRIMARY KEY (image_type_id),
CONSTRAINT FK_Foo_ImageTypes FOREIGN KEY (image_type_id, image_type) REFERENCES Image_Types (image_type_id, image_type)
)
CREATE TABLE Bar (
image_type_id INT NOT NULL,
image_type CHAR(3) NOT NULL CONSTRAINT CHK_Bar_image_type CHECK (image_type IN ('Bar')),
some_bar_specific_column VARCHAR(100) NOT NULL,
CONSTRAINT PK_Bar PRIMARY KEY (image_type_id),
CONSTRAINT FK_Bar_ImageTypes FOREIGN KEY (image_type_id, image_type) REFERENCES Image_Types (image_type_id, image_type)
)
CREATE TABLE Images (
image_id INT NOT NULL,
image_type CHAR(3) NOT NULL CONSTRAINT CHK_Images_image_type CHECK (image_type IN ('Foo', 'Bar')),
image_type_id INT NOT NULL,
CONSTRAINT PK_Images PRIMARY KEY (image_id),
CONSTRAINT FK_Images_ImageTypes FOREIGN KEY (image_type_id, image_type) REFERENCES Image_Types (image_type_id, image_type)
)
An alternative approach, if your RDBMS supports it, is to use filtered indexed views on Image_Types and then have your FKs in Foo and Bar point at those views. That allows you to eliminate the image_type column from some of your tables. Some people prefer the above pattern even when filtered indexed views are available though.