[SQL] A form of inheritance with PostgreSQL

[Greg Toombs]

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