Pid: identification number for materials.
Pname: name for materials.
Sname: name for supplier.
Sid: identification for supplier.
Find all the identification numbers for supplier are provided by Tainan supplier but the lack of part can be provided by Taipei supplier.
The answer is:
WHERE NOT EXISTS(
WHERE Catalog.sid= Supplier.sid AND NOT (city='Tainan' or city='Taipei'))
I can't understand this sentence:
NOT (city='Tainan' or city='Taipei'))
The question already says all the identification number are provided by Tainan supplier and why does the sentence uses NOT.
Thank you very much.