Recovering types through an Isa relationship

Question

Hi! My database has 7 tables: one is the parent (feeds) and the other 3 are the children (public feeds, private feeds, generated feeds) of an isA relationship. There is a table "subscriptions" with a foreign key on the feeds table. Each user can subscribe a feed of any type. The problem is that the view for each type of feed is different, that means I need to generate different links for each type of feed subscribed. Under the current scheme I need to perform 3 queries to get the type of the feed from the feed id. Is there a better solution to this problem?

Answer 1

You could use a view to pre-join the children to the parents, and to get a consistent result regardless of which child type. For example:

create view feed_links as
select f.feed_name
,      case f.feed_type
            when 'public' then pub.x + pub.y
            when 'private' then pri.z
            when 'generated' then gen.v + gen.w
            end as link
from   feeds f
left outer join public_feeds pub on pub.feed_id = f.feed_id
left outer join private_feeds pri on pri.feed_id = f.feed_id
left outer join generated_feeds gen on gen.feed_id = f.feed_id
where ...;

Or if your feeds table doesn't have a feed_type column (or equivalent):

create view feed_links as
select f.feed_name
,      case when pub.feed_id is not null then pub.x + pub.y
            when pri.feed_id is not null then pri.z
            when gen.feed_id is not null then gen.v + gen.w
            end as link
from   feeds f
left outer join public_feeds pub on pub.feed_id = f.feed_id
left outer join private_feeds pri on pri.feed_id = f.feed_id
left outer join generated_feeds gen on gen.feed_id = f.feed_id
where ...;