Re: MV Keys

vc wrote:
> Jon Heggland wrote:
> > In article <1141393487.167645.153600_at_v46g2000cwv.googlegroups.com>,
> > boston103_at_hotmail.com says...
> > >
> > > Saying that the date is not scalar is akin to claiming that 254 is not
> > > a scalar because it consists of units, tens and hundreds ;)
> >
> > You say that as if it were a silly thing to claim. :)
>
> Is it not ? Forgetting about time zones, the date is just a natural
> number in a peculiar notation (or if one wants to be picky, a subtype
> thereof). Is the natural number a compound type ?
>
> > --
> > Jon

This is partially true. It depends of the meaning of the date and on the design of the database.

1.
If you think that the date means "number-of-days" between the year of Christ's birth and this date, then the date is "number-of-days" and you are right - the date is scalar. (same thing with a date_time_stamp and "number of secunds"). In this case you can design for example an entity E1 (A,B,C,date) and use the attribute date for Key. However the attributes A,B,C can have different "number-of-days"; their "datefrom/to/during/etc" can be different; their since when "time-status" has different meaning regarding "numbers-of-days". So we should be careful with date as Key. I wrote about this in more details on my web site: www.dbdesign10.com , 2.4. under 2. and example 2.5

2.
I can choose that year, month, day, hour etc has meaning which doesn't have any relation to "number-of-days" and to the first day. It depends on application. In this case I can design an entity E2 (A, B, C, Year, Month, Day, Hour, Min, Sec). Here "numbers-of-days" or date_time_stamp can't be Key because they aren't the attributes of the entity. Even more, here we don't have an attribute date. Now we can derive date as a compound data (for some programming).

Vladimir Odrljin Received on Mon Mar 06 2006 - 18:20:05 CET