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SQL Problems and solutions S. I. Moiseenko

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Common mistakes in solving SELECT statement exercises

Tips and solutions

Exercise #18 (tips and solutions)

In the following solution

Console

Execute

SELECT c.maker, a.priceA price
FROM (SELECT MIN(price) priceA 
 FROM Printer 
 WHERE Color ='y'
 ) a INNER JOIN 
 Printer b ON a.priceA = b.price INNER JOIN 
 Product c ON b.model = c.model;

subquery defines lowest price for color printers with subsequent joining with Printer table on this price to find all printers having such a price. Finally, joining with Product table gives us the makers of these printers.

Surely, joining on price might be replaced by simple comparison:

WHERE price = (SELECT MIN(price) priceA 
 FROM Printer 
 WHERE Color ='y'
 )

A mistake in the query is not related to said above, but to the fact that the solution determines ANY printers which have price coincident with a minimal price for color printers. Database includes corresponding items. As a result, we obtain

maker	price
D	270.0
A	270.0

Meanwhile, correct answer gives only one row:

maker	price
D	270.0

Second row is not related to color printer. You can be easyly convinced of it if you'll include additional columns in SELECT clause:

SELECT c.maker, a.priceA price, color, b.type

which give

maker	price	color	type
D	270.0	y	Jet
A	270.0	n	Matrix

Second mistake is in possible duplicates, as a maker may have several printers having the same price.

To return to discussion of exercise #18

To solve a problem on SQL-EX.RU

join operations comparison predicates duplicates exercise 18

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SQL Problems and solutions

S. I. Moiseenko

Common mistakes in solving SELECT statement exercises

Tips and solutions

Exercise #18 (tips and solutions)