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SQL Problems and solutions S. I. Moiseenko

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Typical mistakes at the solution of 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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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

Tips and solutions

Exercise #18 (tips and solutions)