Question 18

${\sigma }_{A\ne B}(E)\text{ est une abreˊviation pour }E-{\sigma }_{A=B}(E),\text{ ouˋ }A,B\in \text{sort}(E)$ ${\pi }_{\{W\#\}}(WAREHOUSES)-{\pi }_{\{W\#\}}({\sigma }_{Color\ne red}(STOCK))$

@allow <>
% base de données question 4
@let WAREHOUSES = @relation [W#, Address, City]{
  < D1, "6, Rue de l’Eglise", Mons     >
  < D2, "18, Place du Parc",  Mons     >
  < D3, "18, Place du Parc",  Chimay   >
  < D4, "5, Avenue Louise",   Enghien  >
}
 
@let STOCK = @relation [W#, Product, Color, Qty]{
  < D1, hinge,   yellow,  200 >
  < D1, hinge,   blue,    150 >
  < D2, lock,    blue,    100 >
  < D2, hinge,   yellow,  200 >
  < D2, handle,  red,     100 >
  < D4, hinge,   red,     150 >
  < D4, lock,    red,     600 >
}
 
 
@let result = @project {W#} (WAREHOUSES) - @project{W#} (@select {Color<>'red'} STOCK)
@print result