Exercise #23 (tips and solutions)

To solve this exercise is generally used two approaches: first, the join and selecting the required row and second, check for hit maker in the two lists. The next incorrect solution implements the first approach:

Solution 4.11.1

SELECT DISTINCT a.maker
FROM Product a 
    LEFT JOIN PC b ON a.model = b.model 
              AND b.speed> =750 
    LEFT JOIN Laptop c ON a.model = c.model 
                  AND c.speed> =750
WHERE NOT (b.model IS NULL 
       AND c.model IS NULL
          );

This is another variant on the theme of “something one”. Indeed, the model is unique, ie it is either a PC or laptop. Next rows are obtained as result of external joins:

maker model (PC) NULL

or

maker NULL model (laptop)

However, there can be no rows such as:

maker model (PC) model (laptop)

because the join is performed by model number.

As a result, the list contains only makers who make only one product type with the desired characteristics. However, the correct solution is simply adding the grouping by maker and model counting. We suggest you do it yourself.

Solution 4.11.2

To demonstrate the second approach, consider the following variant:

SELECT DISTINCT t.maker
FROM Product t
WHERE (t.model IN (SELECT model
                    FROM PC
                    WHERE speed >= 750
                    ) 
    OR t.model IN (SELECT model
                    FROM Laptop
                    WHERE speed >= 750
                    )
      ) 
    AND EXISTS (SELECT *
                FROM Product
                WHERE Product.maker = t.maker 
                  AND Product.type='PC'
                ) 
    AND  EXISTS (SELECT *
                FROM Product
                WHERE Product.maker = t.maker 
                    AND Product.type='Laptop'
                );

which can be read as follows: find a manufacturer that produces the PC with at least 750 MHz or a laptop computer with at least 750 MHz, with a given maker must produce both PCs and laptops. Undeniable progress of this solution compared with the decision 1.15.1 is that the findings of makers, such as PC and laptop computers. However, this solution allows for an option when a maker produces only a PC at speeds over 750 MHz, while all of its laptop computers have a speed of less than 750 MHz, and vice versa.

To return to discussion of exercise #23

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