Re: a union is always a join!

"Walter Mitty" <wamitty_at_verizon.net> wrote in message news:EGNvl.685$SU3.443_at_nwrddc02.gnilink.net...
>
> "Brian Selzer" <brian_at_selzer-software.com> wrote in message
> news:c4Fvl.26433$ZP4.2289_at_nlpi067.nbdc.sbc.com...
>>
>
>> As an exercise, try to write a set-based update trigger (a statement
>> trigger in Oracle) on a table with only a composite natural key. You'll
>> find that when a column in the key may be the target of an update, that
>> you can't just join deleted to inserted (old to new in Oracle) to
>> determine exactly what is different. (That's one of the main reasons
>> Oracle and many other implementations support row triggers.)
>>
>>
>
> The idea behind entity keys is that they are immutable. When analyzing
> the universe of discourse into entities, it's important to discover a
> reliable identifier for each entity. In practice, it's sometimes
> necessary to synthesize identifiers, because the identifiers in common use
> are unsuitable. An example might be the use of common names for people at
> the time when information systems were first being computerized.
>
> Foreign keys are immutable during the period of time that they refer to
> the same thing, for the same reason that entity keys are immutable. It
> follows that foreign keys should not have to be updated as a cascade from
> the update of the entity keys they refer to. An update to a foreign key
> that represents negation of the old relationship and assertion of a new
> one ought to be expressble as a delete followed by an insert. I could be
> wrong on this, as I haven't completed the exercise you proposed.
>
> The tables I have built with only composite natural keys have all been
> relationship tables, and not entity tables. In these cases, the composite
> key that identifies rows in the relationship table are also foreign keys
> that reference entities.
>

An instance of a relationship isn't any less a thing than an instance of an entity, so I think we should try to avoid all of the baggage associated with the terms "entity" and "relationship."

> By the way, are you claiming that all attributes are mutable?
>

I'm claiming that whether an attribute is mutable or not is orthogonal to the Relational Model. What about a relation for which the entire heading is the key? Shouldn't it still be possible to issue an update? Received on Tue Mar 17 2009 - 23:56:38 CET