> On Fri, 2002-09-06 at 07:37, Curt Sampson wrote: > > On 5 Sep 2002, Hannu Krosing wrote: > > > > > Suppose you have a table CITIZEN with table-level constraint IS_GOOD > > > which is defined as kills_not_others(CITIZEN). and there is table > > > CIVIL_SERVANT (..) UNDER CITIZEN. Now you have just one table MILITARY > > > (...) UNDER CIVIL_SERVANT, where you have other criteria for IS_GOOD.... > > > > This I very much disagree with. > > > > In most object-oriented languages (Eiffel being a notable exception, IIRC), > > you can't specify constraints on objects. But in a relational database, > > you can specify constraints on tables, and it should *never* *ever* be > > possible to violate those constraints, or the constraints are pointless. > > That's not how real world (which data is supposed to model) operates ;) > > As Greg already pointed out, there are two kinds of constraints - > database integrity constraints (foreign key, unique, not null, check), > which should never be overridden and business-rule constraints which > should be overridable in child tables. > > one can argue that the latter are not constraints at all, but they sure > look like constraints to me ;) > > To elaborate on Gregs example if you have table GOODS and under it a > table CAMPAIGN_GOODS then you may place a general overridable constraint > valid_prices on GOODS which checks that you dont sell cheaper than you > bought, but you still want sell CAMPAIGN_GOODS under aquiring price, so > you override the constraint for CAMPAIGN_GOODS.
What that tells me is that the constraint, valid_prices, shouldn't have been on GOODS in the first place. If it is not a legitimate constraint for the children, then it is not a legitimate constraint for the parent. In human inheritance, if you marry someone with "funny coloured skin," you don't get to choose that your children won't have "funny coloured skin." That's a pretty forcible "constraint." :-). For the GOODS situation, the constraint ought not to be on GOODS in the first place. There ought to be a table ORDINARY_GOODS, or some such thing, to which the constraint applies, and from which CAMPAIGN_GOODS will _not_ be inheriting. > > So if I have a constraint that says, "no rows appearing in this > > table will ever violate constraint X," and then you go and create > > a way of inserting rows into that table that violate that constraint, > > I think you've just made the database into a non-relational database. > > SQL standard constraints should be non-overridable. I still think that > Constraint triggers should be overridable/dynamic. > > Or maybe it is better to just make the check function should be > dynamically dispatched, so the constraint will always hold, it just can > mean different things for different types. Or maybe if someone is doing an Object Oriented design, and making extensive use of inheritance, they'll need to apply constraints in a manner that allow them to be properly inherited. -- (concatenate 'string "aa454" "@freenet.carleton.ca") http://cbbrowne.com/info/ If a cow laughed, would milk come out its nose? ---------------------------(end of broadcast)--------------------------- TIP 3: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to [EMAIL PROTECTED] so that your message can get through to the mailing list cleanly
