Re: insert to projection

On Sep 4, 9:58 am, paul c <toledobythe..._at_oohay.ac> wrote: > ... We can delete from projection ...

Are you suggesting projection being updatable in respect to deletion? I fail to see this, here is my worksheet:

Let x be the base relation,
z an empty relation with subset of attributes of x, y = x v z the projection of x onto attributes of z, dx decrement of x,
dy decrement of y.

Counterexample:

dx = [p q]

      1  a
      2  a

      2
;

z = [p]
;

x = [p q]

     1  a
     1  b
     2  a

QBQL assertion that generated it (against Figure1.db database):

z != []          &        -- that is z != DUM. Introduced after QBQL
                            -- found counterexample with z = []
z ^ [] = z       &       -- z is an empty relation (aka header)
--x v z = y      &       -- View def: y is projection of x onto z
                              -- Substituted y with (x v z) everywhere
                              -- to make the counterexample search
faster
x ^ [] = dx ^ [] &       -- x and dx have the same header
(x v z) ^ [] = dy ^ [] &       -- y and dy have the same header
z > x ^ []       &       -- Atributes of z are contained in attributes
of x
dx < x &                 -- decrement relation containment
dy < x v z &                 -- ditto
(x ^ dx') v z = (x v z) ^ dy'  -- View definition after update
-> dx = dy ^ x.         -- decrement of the base in terms of
                              -- decrements of the view and base

If you are uncomfortable with deletion of more than one tuple at once, then here is modified single tuple counterexample:

dx = [p q]

      1 a
;

dy = [p]
;

z = [p]
;

x = [p q]

     1  a
     1  b
     2  a