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Exercise #56 page 4

Solution 3.12.4

Console
Execute
  1. SELECT class, SUM(sunks) sunks 
  2. FROM (SELECT cl.class, 1 sunks 
  3.       FROM Classes cl LEFT JOIN 
  4.            Ships sh ON cl.class = sh.class INNER JOIN 
  5.            Outcomes ou ON ou.ship = sh.name OR
  6.                           ou.ship = cl.class 
  7.            WHERE result='sunk'
  8.        UNION
  9.        SELECT DISTINCT class, 0 sunks 
  10.        FROM classes
  11.       ) tab
  12. GROUP BY class;

The first of united queries in the FROM clause gives the following row in the result set for each sunken ship:

class            1

In the main query, these «units» are being summed. But as UNION eliminates duplicates, finally we obtain either 1 (from this first query) or 0 (from second query which considers the classes which are not having the sunken ships) for any quantity of the sunken ships in a class.

If to unite by means of UNION ALL, we'll get to the solution 3.12.2 which contains a similar mistake.

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