Exercise #23 page 3
To not be proofless, I'll show the results of original query (solution 1.15.2) with an extension of output columns set:
Let's consider a couple of rows from the result set:
As you can see, model 1121 (PC) belongs to the maker B, but model 1752 (laptop) – to the maker A. So, we don't have a reason to consider that both these makers satisfy the task conditions.
Uniting of demanded models of the PCs and laptops in one set gives only illusion that we receive both types:
As a result, the list of manufacturers for which there is at least one row in a set from FROM clause will be received. Below shorter variant of the same mistake.