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Typical mistakes at the solution of SELECT statement exercises

Database «Ships»

Exercise #39 page 3

Solution 3.4.3

It's said but true, the following query was accepted by the system until the last days:

Console

Execute

SELECT s.name
FROM Ships s JOIN 
 Outcomes o ON s.name = o.ship JOIN 
 Battles b ON o.battle = b.name
GROUP BY s.name 
HAVING COUNT(s.name) = 2 AND 
 (MIN(result) = 'damaged' OR 
 MAX(result) = 'damaged'
 )
UNION 
SELECT o.ship
FROM Classes c JOIN 
 Outcomes o ON c.class = o.ship JOIN 
 Battles b ON o.battle = b.name 
WHERE o.ship NOT IN (SELECT name 
 FROM Ships
 ) 
GROUP BY o.ship 
HAVING COUNT(o.ship) = 2 AND 
 (MIN(result) = 'damaged' OR 
 MAX(result) = 'damaged'
 );

Firstly, the queries, which perform joining the ships taken part in battles (Outcomes table) with the Ships table and Classes table accordingly, are being united. By the way, the predicate

o.ship NOT IN (SELECT name FROM Ships)

in the second query is apparently unwanted, because UNION will exclude all possible duplicates.

These joins are not just surplus, they are wrong, as the description of the database says that in the Outcomes table can be ships that are absent from Ships table. That is, if not the leading ship is found, which is absent from the Ships table and which satisfies the task terms, then it will not get into the resulting set of the aforecited query.

Secondly, the predicate

HAVING COUNT (o.ship) = 2

confines possible variants with only two ship battles. And why can't the ship take part in more than two battles? It was not obligatory sunk after it had been damaged. And he could participate in battles and before damage (for example, with result "ok"). Then, if in the following and the last battle of a ship she has been damaged, the query 3.4.3 will deduce this ship though it and does not respond conditions of the task.

Thirdly, I don't quite understand the condition:

(MIN(result) = 'damaged' OR MAX(result) = 'damaged')

Pages

mistakes Outcomes Battles

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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

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SQL Problems and solutions

S. I. Moiseenko

Typical mistakes at the solution of SELECT statement exercises

Database «Ships»

Exercise #39 page 3