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Find street number of the longest street in each city in Canada

Relevant relations:

City(city-name,country-name,area,population) Street(stno,city-name,length)

The keys are bolded

My tuple relational calculus set is defined:

 {l.stno | STREET(l) and forAll c (CITY(c) and c.country-name = "Canada") -> doesn't Exist s (STREET(s) and s.city-name=c.city-name and s.length > l.length)}

I think this searches for the 1 longest street from all Canadian cities instead. How can I make this correct?

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