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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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