looking for a term

the latest glossary reminded me of a ..uestion (sorry, the key on my keyboard that stands for the letter between 'p' and 'r' is broken) that i've been trying to phrase for some time.

here goes:

E:
emp# name addr

1	Ted     555 ...
2	Bill	23 ...

let's say emp# is the only candidate key. if we keep nulls out of things, i'd say the above '3-d'/ternary relation has more meaning than the 2-d/binary relations i could project from it, ie. N{emp#,name} and A{emp#,addr}.

trying to use common lingo, i'd characterize the extra meaning as an implied constraint, for example, "if an employee has an employee number, he must have a name and he must have an address". whereas the same constraint isn't implied if my schema contains only the N and A relations.

two ..estions:

1. is there a well-known term that describes this situation?
2. if i natural join N and A, producing a result that matches the contents of E, am i introducing the constraint in some sense? maybe this is the same as asking if i take the two projections of E above have i dispensed, in some sense, with that constraint?

p Received on Sat Jul 09 2005 - 20:39:33 CEST