--9

PROJECT [SNAME] (SELECT [SCITY = 'Houston'] (SUPPLIER));

-- 16

(Part JOIN (PROJECT [PNUM] (SELECT [SNUM = 'S1'] (SUPPLY))))
UNION
(Part JOIN (PROJECT [PNUM] (SELECT [SNUM = 'S2'] (SUPPLY))));

-26

Active supplier numbers:
PROJECT [SNUM] (Supply);

All supplier numbers:
PROJECT [SNUM] (Supplier);

Answer:

PROJECT [SNUM] (Supplier)
MINUS
PROJECT [SNUM] (Supply);

- 30

All part numbers by S1:
PROJECT [PNUM] (SELECT [SNUM = 'S1'] (SUPPLY));

All supplier numbers:
PROJECT [SNUM] (SUPPLIER);

All (SNUM, PNUM) pairs, where PNUM is supplied by S1.
(PROJECT [PNUM] (SELECT [SNUM = 'S1'] (SUPPLY)))
JOIN
(PROJECT [SNUM] (SUPPLIER));

(PROJECT [SNUM] (SUPPLIER))
TIMES
(PROJECT [PNUM] (SELECT [SNUM = 'S1'] (SUPPLY)));

Supply without quantity:
PROJECT [SNUM, PNUM] (SUPPLY);


Parts (supplied by S1) that are not supplied by supplier:


((PROJECT [SNUM] (SUPPLIER))
TIMES
(PROJECT [PNUM] (SELECT [SNUM = 'S1'] (SUPPLY))))
MINUS
(PROJECT [SNUM, PNUM] (SUPPLY));

SNUM that does not supplied some parts (supplied by S1):
These SNUM should not be in the result:

PROJECT [SNUM]
(((PROJECT [SNUM] (SUPPLIER))
TIMES
(PROJECT [PNUM] (SELECT [SNUM = 'S1'] (SUPPLY))))
MINUS
(PROJECT [SNUM, PNUM] (SUPPLY)));


ANswer:
(PROJECT [SNUM] (SUPPLIER))
MINUS
PROJECT [SNUM]
(((PROJECT [SNUM] (SUPPLIER))
TIMES
(PROJECT [PNUM] (SELECT [SNUM = 'S1'] (SUPPLY))))
MINUS
(PROJECT [SNUM, PNUM] (SUPPLY)));