Re: What to call this operator?

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?

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?

• Jan Hiddesr