Define the average size of the PC hard drive for each maker that also produces printers.Result set: maker, average capacity of HD.
Track the steps of the solution to the exercise and discover mistakes.
1. Determining all manufacturers who produce printers.
2. Deducing the size of a hard disk and its manufacturer for each personal computer.
3. Choosing only the rows from item 2 which include the manufacturer found in item 1.
4. In the final solution we receive average values on the basis of query from item 3.