Exercise #18 (tips and solutions)

In the following solution

SELECT c.maker, a.priceA price
FROM (SELECT MIN(price) priceA
      FROM Printer
      WHERE Color ='y'
      ) a 
    INNER JOIN Printer b ON a.priceA = b.price 
INNER JOIN Product c ON b.model = c.model;

subquery defines lowest price for color printers with subsequent joining with Printer table on this price to find all printers having such a price. Finally, joining with Product table gives us the makers of these printers.

Surely, joining on price might be replaced by simple comparison:

WHERE price = (SELECT min(price) priceA
               FROM Printer
               WHERE Color ='y'
              )

A mistake in the query is not related to said above, but to the fact that the solution determines ANY printers which have price coincident with a minimal price for color printers. Database includes corresponding items. As a result, we obtain

Meanwhile, correct answer gives only one row:

Second row is not related to color printer. You can be easily convinced of it if you’ll include additional columns in SELECT clause:

SELECT c.maker, a.priceA price, color, b.type

which give

Second mistake is in possible duplicates, as a maker may have several printers having the same price.

To return to discussion of exercise #18

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