Re: Bags versus sets; are they needed?

> First Name Last Name Weight
> ---------- --------- ------
> Sam Jones 135.88
> Kara Jones 135.88
>
> What is projection to <Last Name, Weight> is? Would it contain two
> records or one? Does the answer depend upon precision?

I think that issues of measurement and projection get very interesting indeed. I would suggest that when the original measurement is made of the weight, the weight as measured is a projection, made by the scales, from the actual weight to the measurement space. There are at least two sources of error in the scale, errors due to inaccuracy and errors due to lack of precision. Once the measure is stored in a computer, there is another projection that has taken place: the projection of the weight as measured onto the subset of the real numbers that the numeric datatype can represent.

Any one of these projections can lose information. The lost information may or may not be of value to the originator of the query. Testing two real values for "equality" is always dicey. Look over any standard text on numerical analysis for a more complete treatment.

--
Regards,
    David Cressey
    www.dcressey.com