Foreign key pointing to different tables
- by Álvaro G. Vicario
I'm implementing a table per subclass design I discussed in a previous question. It's a product database where products can have very different attributes depending on their type, but attributes are fixed for each type and types are not manageable at all. I have a master table that holds common attributes:
product_type
============
product_type_id INT
product_type_name VARCHAR
E.g.:
1 'Magazine'
2 'Web site'
product
=======
product_id INT
product_name VARCHAR
product_type_id INT -> Foreign key to product_type.product_type_id
valid_since DATETIME
valid_to DATETIME
E.g.
1 'Foo Magazine' 1 '1998-12-01' NULL
2 'Bar Weekly Review' 1 '2005-01-01' NULL
3 'E-commerce App' 2 '2009-10-15' NULL
4 'CMS' 2 '2010-02-01' NULL
... and one subtable for each product type:
item_magazine
=============
item_magazine_id INT
title VARCHAR
product_id INT -> Foreign key to product.product_id
issue_number INT
pages INT
copies INT
close_date DATETIME
release_date DATETIME
E.g.
1 'Foo Magazine Regular Issue' 1 89 52 150000 '2010-06-25' '2010-06-31'
2 'Foo Magazine Summer Special' 1 90 60 175000 '2010-07-25' '2010-07-31'
3 'Bar Weekly Review Regular Issue' 2 12 16 20000 '2010-06-01' '2010-06-02'
item_web_site
=============
item_web_site_id INT
name VARCHAR
product_id INT -> Foreign key to product.product_id
bandwidth INT
hits INT
date_from DATETIME
date_to DATETIME
E.g.
1 'The Carpet Store' 3 10 90000 '2010-06-01' NULL
2 'Penauts R Us' 3 20 180000 '2010-08-01' NULL
3 'Springfield Cattle Fair' 4 15 150000 '2010-05-01' '2010-10-31'
Now I want to add some fees that relate to one specific item. Since there are very little subtypes, it's feasible to do this:
fee
===
fee_id INT
fee_description VARCHAR
item_magazine_id INT -> Foreign key to item_magazine.item_magazine_id
item_web_site_id INT -> Foreign key to item_web_site.item_web_site_id
net_price DECIMAL
E.g.:
1 'Front cover' 2 NULL 1999.99
2 'Half page' 2 NULL 500.00
3 'Square banner' NULL 3 790.50
4 'Animation' NULL 3 2000.00
I have tight foreign keys to handle cascaded editions and I presume I can add a constraint so only one of the IDs is NOT NULL.
However, my intuition suggests that it would be cleaner to get rid of the item_WHATEVER_id columns and keep a separate table:
fee_to_item
===========
fee_id INT -> Foreign key to fee.fee_id
product_id INT -> Foreign key to product.product_id
item_id INT -> ???
But I can't figure out how to create foreign keys on item_id since the source table varies depending on product_id. Should I stick to my original idea?
