Exercise #15

This incorrect solution is the result of a superficial knowledge of the database schema:

Solution 1.11.1

SELECT DISTINCT t.hd
FROM PC t
WHERE EXISTS (SELECT *
              FROM PC
              WHERE pc.hd = t.hd
                AND pc.model <> t.model
            );

This query gets such PCs for which another PC model having the same hard drive capacity exists. The mistake lies in the intuitive assumption of the model uniqueness in the PC table. However, as mentioned earlier, models are unique in the Product table only; in other tables, duplicates can occur. This fact renders the query under review wrong, since it excludes identical models with the same hard drive capacity from consideration.

T&S

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