Multiple Foreign Keys

Question

My database designs have previously been rather linear, so I'm getting stumped on what is probably a very easy to solve problem.

I have a table of "POSTS", which contain posts which may be a child of either a "CATEGORY" or a "TOPIC". What would be the best way to define the foreign key(s) for the "POSTS" table?

I suppose I could have a column named POST_CATEGORY_ID and a field named "POST_TOPIC_ID, which could be nullable, but this just doesn't sound right. Surely there is a simple solution that I'm missing!

Answer 1

You could do many-to-many relationships between POST and CATEGORY and POST and TOPIC:

POST
----
ID
Text ...
...

CATEGORY
--------
ID
Name

TOPIC
-----
ID
Name

POST_CATEGORY
-------------
POST_ID (FK)
CATEGORY_ID (FK)

POST_TOPIC
----------
POST_ID (FK)
TOPIC_ID (FK)

This way, a post can be associated with any number of categories and topics.

Answer 2

You're on the right track with nullable POST_CATEGORY_ID and POST_TOPIC_ID fields. This will model that a post optionally be related to a category and optionally related to a topic.

If this is meant to be exclusive and mandatory, you will need to add a check constraint that either is null, but not both.

Answer 3

I think a foreign key declaration can only refer to a single table:

FOREIGN KEY(CATEGORY_ID) REFERENCES CATEGORY(CATEGORY_ID);
FOREIGN KEY(TOPIC_ID) REFERENCES TOPIC(TOPIC_ID);

If I'm correct, you'll have to have two foreign keys, one for the CATEGORY table and another for TOPIC, and both need to be nullable.

Answer 4

How about having the categories and topics in the same table

create table topics_categories( id number, description varchar2(100), item_type char(1)); --C or T

Then a single foreign key to topics_categories

Answer 5

The best way is to use Inheritance. You will have two specializations of "Posts": "Posts_Category" and "Posts_Topic" Both have "post_id" (which relates to the parent table "Posts") and another field:

"Category_ID" ("Posts_Category")

"Topic_ID" ("Posts_Topic")

If this sounds confuse look at Doctrine (PHP ORM) at their documentation: http://www.doctrine-project.org/documentation/manual/1_0/en/inheritance

Answer 6

If you want to define foreign keys in the database, the solution that you're suggesting sounds right. I also suggest writing some code in a trigger to make sure that both fields aren't null.