When you imported this “Northwind” database column names were imported in CamelCase – your import must’ve added double quotes to column identifiers to create table queries.
This is rather unfortunate, as this would cause that you’d have to quote them also in all queries, like:
select "City" from customers;
To remain sane I’d suggest you to rename all columns to lower case. This way it wouldn’t matter what case you use, as Postgres converts all unquoted identifiers to lower case automatically. Then any of this would work:
select city from customers;
select City from Customers;
SELECT CITY FROM CUSTOMERS;