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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Pages

mistakes laptop PC WHERE exercise 23

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Exercise #23 page 1