02:28

	Content	Login

SQL Problems and solutions S. I. Moiseenko

Content

SQL console

back go forward

Typical mistakes at the solution of SELECT statement exercises

Database «Ships»

Exercise #56 page 4

Solution 3.12.4

Console

Execute

SELECT class, SUM(sunks) sunks 
FROM (SELECT cl.class, 1 sunks 
      FROM Classes cl LEFT JOIN 
           Ships sh ON cl.class = sh.class INNER JOIN 
           Outcomes ou ON ou.ship = sh.name OR
                          ou.ship = cl.class 
           WHERE result='sunk'
       UNION
       SELECT DISTINCT class, 0 sunks 
       FROM classes
      ) tab
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.

Pages

mistakes UNION exercise 56

Previos [Exercise #55]

[Exercise #57] Next

Last added:

Tags

aggregate functions ALL ASCII AVG Battles Bezhaev Bismarck C.J.Date Cartesian product CASE cast CHAR CHARINDEX classes COALESCE common table expressions comparison predicates Computer firm CONVERT correlated subqueries COUNT CTE data type conversion database schema DATEDIFF DATEPART DATETIME DELETE DISTINCT EXCEPT exercise (-2) exercise 19 exercise 23 exercise 37 exercise 39 exercise 46 exercise 54 exercise 55 exercise 56 exercise 57 exercise 7 exercise 70 exercise 8 exercises EXISTS FROM FULL JOIN GROUP BY grouping Guadalcanal More tags

The book was updated
yesterday

SQL Problems and solutions

S. I. Moiseenko

Typical mistakes at the solution of SELECT statement exercises

Database «Ships»

Exercise #56 page 4