# Re: A different definition of MINUS, Part 3

Date: Sun, 21 Dec 2008 12:40:12 -0800 (PST)

Message-ID: <4052efb5-66d4-4e12-9f44-4bc289c189a0_at_p2g2000prf.googlegroups.com>

On Dec 21, 11:53 am, Bob Badour <bbad..._at_pei.sympatico.ca> wrote:

*> vadim..._at_gmail.com wrote:
*

> > On Dec 21, 11:36 am, vadim..._at_gmail.com wrote:

*>
**> >>x + y = ((x ^ x)' ^ (y ^ y)')'.
**> >>... is tough...
**>
**> > Actually, easy. The right side simplifies to
**>
**> > x + y = (x' ^ y')'.
**>
**> > Applying double negation (provable in RL), we get
**>
**> > (x+y)' = x' ^ y'.
**>
**> > De Morgan, which is again provable in RL. Amazing.
**>
**> Why are you amazed?
*

Two reasons:

1. Relational Algebra is a weaker system than boolean algebra.
Therefore there is no reason to expect BA properties to carry over.
2. All RA operation, not just three can be reduced to a single one.
Here is my worksheet:

Join, is <AND>. It is representable in terms of Sheffer stroke. Projection, is inner union in RL terms. Well, I don't see how to represent inner union in terms of other operations:-( Received on Sun Dec 21 2008 - 21:40:12 CET