Exercise #30 (tips and solutions)
Exercise #30 (tips and solutions)
To solve a problem of doubling, trebling of results, it is enough to execute corresponding grouping over each table beforehand, and already then to do joining them. Then this exercise reduces to a exercise 29, the solution of which did not cause issues and consequently is not included in the book.
It not a unique way to solve a problem. Visit a forum of the site to meet other approaches to solution.