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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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