Re: I think my book may be wrong about cardinality, but I'm not sure

From: Gene Wirchenko <>
Date: Tue, 24 Jul 2007 14:21:45 -0700
Message-ID: <>

[Quoted] "David Cressey" <> wrote:

>"beginner16" <> wrote in message


>> Now as far as relational DB goes, don't tables have only binary
>> connections ( second degree relationship )?

>No. You can have a relational table whose key is made up of three foreign
>This implements a ternary relationship.
>Take an airline reservation system. A single instance of a reservation
>might be made up of three keys: passenger_id, flight_id, and date. Date
>is often not thought of as an "entity", but it can be awfully handy to have
>a table with one row per date, with all the attributes of the date.
>Note that without any one of the three items, the reservation is ambiguous.
>There is another way to model this situation, but I've chosen the way that
>illustrates reservation as a ternary relationship.

     Another ternary is Part a is sold to customer b and supplied from warehouse c.



Gene Wirchenko

Computerese Irregular Verb Conjugation:

     I have preferences.
     You have biases.
     He/She has prejudices.
Received on Tue Jul 24 2007 - 23:21:45 CEST

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