通过ISA关系恢复类型

您可以使用一个视图将子对象预联接到父对象,并获得一致的结果,而不管子对象类型是什么。例如:

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 ...;

或者如果提要表没有提要类型列(或等效列):

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 ...;