Tutorial D with + restrict

I am reading Date's Databases in Depth now, and having trouble with expressions like this (it shoud return supp. numbers with numbers of distinct parts each suppliers supplies):

WITH ( SP RENAME ( SNO AS X ) ) AS R :
EXTEND ( S { SNO } ) ADD ( COUNT ( R WHERE X = SNO ) AS NP ) what troubles me is thist inner "R WHERE X = SNO" part

It puzzles me how can R "know" value of SNO attribute? Is it just repeatedly evalueated for each tuple of S{SNO}? Seems so, but this scenario is not covered in the explanation of RESTRICT operator.
Is my conceptual algorithm correct?

Thanx,
Strider5 Received on Thu Mar 30 2006 - 09:24:46 CEST