Re: relational algrabra division?
From: James K. Lowden <jklowden_at_speakeasy.net>
Date: Thu, 11 Dec 2014 21:13:16 -0500
Message-Id: <20141211211316.377bdab6.jklowden_at_speakeasy.net>
>
> Really? What kind?
Date: Thu, 11 Dec 2014 21:13:16 -0500
Message-Id: <20141211211316.377bdab6.jklowden_at_speakeasy.net>
On Thu, 11 Dec 2014 13:04:27 -0500
ruben safir <ruben_at_mrbrklyn.com> wrote:
> What class of query is this called and how is it generlized
> mathmatically.
A join is called a "cross product" and may be thought of as multiplication. Relational division is its inverse:
C = A × B B = C ÷ A
It is not well represented in SQL, unfortunately.
>> sometimes occurs in database applications.
>
> Really? What kind?
> (e) Retrieve the names of employees who work on every project
That kind!
--jkl Received on Fri Dec 12 2014 - 03:13:16 CET