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:

  1. SELECT AVG(speed) AS avg_speed
  2. FROM PC
  3. WHERE speed IN (SELECT speed
  4. FROM PC, Product
  5. WHERE product.model = PC.model AND
  6. maker='A'
  7. );

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.

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