Exercise #13 (computer firm) | Interactive tutorial on SQL | SQL-tutorial.ru

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	Typical mistakes at the solution of SELECT statement exercises Database "Computer firm" Exercise #13
	Find out the average speed of the PCs produced by maker A. Solution 1.10.1. Usual mistake of beginners is applying of unsuitable learned language statements. Here is typical example: Console Execute SELECT AVG(speed) AS avg_speed FROM PC WHERE speed IN (SELECT speed FROM PC, Product WHERE product.model = PC.model AND maker='A' ); SELECT AVG(speed) AS avg_speed FROM PC WHERE speed in (SELECT speed FROM PC, Product where product.model = PC.model AND maker='A' ); Scheme In the subquery of WHERE clause the speed values of PC's processors, produced by A producer, are extracting. Then average speed are calculating for all PC with processor speed equal to any value from list extracted by subquery. As a result a speed of the B's PC producer will be taken into account if speed coincides with speed of any A's PC producer. The result will be only correct if PC producers have models with unique speed of PC processor. To solve the problem on SQL-EX.RU mistakes subquery AVG average value exercise 13

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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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