Marshall Spight wrote:
> Jon Heggland wrote:
>
>>In article <1120230564.593323.241990_at_g43g2000cwa.googlegroups.com>,
>>marshall.spight_at_gmail.com says...
>>
>>>>>With join, all keys are preserved.
>>>>
>>>>Umm... they are? What do you mean by "preserved"? Perhaps I
>>>>misunderstand you, but a key of one of the operands is not necessarily a
>>>>key of the result.
>>>
>>>Okay. What rule would you propose?
>>
>>I'm not sure I understand you. Surely the keys of a join result are
>>determined by logic, not by rules one might propose?
>
> You're quibbling over terminology. What rule, derived via
> logic, would you propose? What's the right answer? Phrase
> it any way you like; I just want to know what the correct
> answer is.
The rule is that if we take the natural join of R and S then we can
derive a candidate key K for the result if K is a candidate key of both
R and S. Is that what you wanted to hear?
Received on Sat Jul 02 2005 - 14:45:47 CDT
