Question

I’m attempting to model objects in MySQL using what is commonly (?) referred to as Class Table Inheritance.

See here: http://martinfowler.com/eaaCatalog/classTableInheritance.html

Essentially I have three tables:

Items (item_id,posX,posY,sort)

Items_Text (item_id,text)

Items_Image (item_id, url)

When I select data I only want to get:

item_id,posX,posY,sort,text

OR

item_id,posX,posY,sort,url

Can anyone suggest a method for doing this?

The only method I can think of is hard coding it based on an additional column “type” in the Items table or some other method of hard coding it into the application logic I’d prefer not to consider. I guess I’m hoping for some complex (or otherwise) method of using joins I’m unfamiliar with?

Thanks,

Ken

Answer 1

You can use left outer join, and check the results for null in the joined ID column, like this:

select
    i.item_id
,   i.posX
,   i.posY
,   i.sort
,   t.item_id
,   t.text
,   u.item_id
,   u.url
from Items i
left outer join Items_Text t on t.item_id=i.item_id
left outer join Items_Image u on u.item_id=i.item_id

When you examine the result set that you get back, you will get non-null in t.item_id only when the item is in the Items_Text, and a non-null in u.item_id only when the item is in the Items_Image. You can use this test to determine the type without having an explicit column.