Hello.<br /><br /> I'm trying to figure out how to nicely implement a C++ class-like system with PostgreSQL. Consider
thefollowing:<br /><br /> Tables Fruit, Apple, Orange<br /><br /> I want to design the foreign key scheme such that
thereare relations between fruit and apple, and fruit and orange, that imply that apple is a fruit, and orange is a
fruit.<br/><br /> I don't want to eliminate the existence of Apple and Orange tables, because there will be columns
specificto both Apple and Orange; if I include these columns in Fruit, then if Fruit is an Orange, the Apple columns
willbe needlessly present in Apple rows.<br /><br /> The different ways of implementing this scheme that I've thought
of(some uglier than others):<br /><br /> - Have Fruit contain foreign keys to both Apple and Orange, and write a check
constraintin Fruit specifying that exactly one of (Apple FK, Orange FK) needs to be non-null. The disadvantage of this
methodis that it isn't exactly loosely coupled. For every other fruit type table I implemented I'd have to go back and
adda foreign key in Fruit.<br /><br /> - Have a foreign key in Apple to Fruit, and in Orange to Fruit; then somehow
createa constraint that imposes uniqueness on the union of foreign keys in both Apple and Orange. To figure out what
typeof fruit a Fruit row is, run a query for foreign keys in Orange and Apple matching the primary key of Fruit. You'd
alsowant to somehow create a constraint that the result of this query should always return exactly one row (perhaps
witha trigger?)<br /><br /> Any advice will be appreciated! As I'm relatively new to Postgre, I might need some help
withthe actual implementation as well.<br /><br /> Thank you.<br /><br /> - Greg<br />
