Re: Thinking about MINUS

From: Marshall <marshall.spight_at_gmail.com>
Date: 18 Jan 2007 21:58:42 -0800
Message-ID: <1169186322.445372.13320@s34g2000cwa.googlegroups.com>

On Jan 9, 8:58 am, Bob Badour <bbad..._at_pei.sympatico.ca> wrote:

> Thus, if joining attributes with similar names and different types is an
> error, it is an error unrelated to the equality comparison. If there is
> an error, it must relate to the data type of the resulting joined
> attribute, which raises the question: "What is the data type of the
> resulting joined attribute?" It is obviously the most specific supertype
> of the types of the joined attributes.

Rather, in a system with a type lattice (subtypes and supertypes) the most specific supertype of the two types would be the result of a union of two relations with a union-incompatibility. This supertype would be able to accommodate values from both subtypes, which it would need to since it's a union. In the case of a join, the resulting
attribute type would be the most general subtype of the two types.

The resulting types might be the supremum or infimum of the type lattice (respectively.) These types are called "top" and "bottom" in type theory; TTM calls them "alpha" and "omega."

Marshall Received on Thu Jan 18 2007 - 23:58:42 CST