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Typical mistakes at the solution of SELECT statement exercises

Database "Computer firm"

Exercise #23 page 1

Find the makers producing at least both a pc having speed not less than 750 MHz and a laptop having speed not less than 750 MHz. Result set: Maker

Query below includes typical mistake peculiar to beginners which solve this exercise.

Solution 1.15.1

Console

Execute

SELECT DISTINCT maker 
FROM product 
WHERE model IN (SELECT model 
 FROM PC 
 WHERE speed >= 750
 ) OR 
 model IN (SELECT model 
 FROM Laptop 
 WHERE speed >= 750
 );

The mistake lies in the fact that the result set will include a producer which manufactures something one: either PC or laptops, because the predicate in WHERE clause will be true if at least one of two conditions linked with OR operator will be satisfied. This doesn't match the task and is fairly discarded by a system.

T&S

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mistakes laptop PC WHERE exercise 23

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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 "Computer firm"

Exercise #23 page 1