Re: Unique Keys

From: Kenneth Downs <firstinit.lastname_at_lastnameplusfam.net>
Date: Fri, 26 Nov 2004 08:12:44 -0500
Message-ID: <cnmj72-7la.ln1_at_pluto.downsfam.net>

Kenneth Downs wrote:

> Tony Andrews wrote:
> 

>> Kenneth Downs wrote:

>>> I would suggest it is more useful to define the equality operator as

>> such:

>>>
>>> where x = (a,b)
>>> and   y = (c,d)
>>>
>>> x = y iff (a <= c <= b) OR (a <= d <= b)
>>>
>>> It is the equality operator definition that makes it a declarative

>> unique

>>> constraint exactly the same as any other, and also makes possible

>> simple

>>> foreign keys.  Now we have:
>>>
>>> CREATE TABLE reservations (
>>>  hotel_room char(4),
>>>  span DATEINTERVAL )
>>>  PRIMARY KEY (hotel_room,span)

>>
>> Yes, but only by subverting the traditional meaning of "equals"! So
>> what do you do now if you want to know if the 2 DATEINTERVALs really
>> are the same?
>>

> 
> Perhaps you would be more comfortable with the OO term?  Operator
> Overloading?
> 

oops, I see I missed your question. You want to know how to tell if a=c and b=d? Well, you seem to be a smart guy, maybe you can suggest something?

-- 
Kenneth Downs
<?php $sig_block="Variable scope? What's that?";?>

Received on Fri Nov 26 2004 - 14:12:44 CET