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Typical errors at solution of exercises.
Exercise 18
S. Moiseenko
Find the makers of the cheapest color printers. Result set: maker, price.
For last days I have received some questions on this exercise and have decided to
write answer in FAQ.
For example, I am asked why the following solution is not adopted by the checking
system.
select product.maker, printer.price
from product, printer
where product.model = printer.model
and printer.price =
(select min( price)
from printer
where color = 'y')
I responded that here there is no check of a condition, that the printer is a color
one. However, it was not appears enough for anybody :-) as I received the second
question: and what about the condition
printer.price =
(select min( price)
from printer
where color = 'y')
?
Last predicate compares the price of EACH printer with a floor price on color ones.
Therefore, if it will appear that NOT color printer (for example, matrix) will have
the price coincident with a minimal price for color printers, the latter will be
in the result set of query considered.
Also do not forget to exclude duplicates.
» Given examples here can be done directly on the website by selecting the check
box “Without checking” on the page with
SELECT exercises .
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