I assume you heard about SET, usually it can be heard in Mathematics and SET operators in SQL has the same meaning as that one as well for UNION and INTERSECT.
SET operators includes
It returns a result set of distinct rows combined from both sides. This means that it will display all the records from both table but will not show the duplicate rows, it’s like as if every record is unique.
My own concept: ((Purchasing + Sales) – Duplicates).
It returns a result set with all rows from both sets. Consider that it display all the records from each of the table. Duplicates will occurs here.
My own concept: (Purchasing + Sales).
It returns only distinct rows that appear in both result sets. The results shows only exist in both table.
My own concept: (Purchasing & Sales)
It returns only distinct rows that appear in the left set but not the right.
My own concept: (Sales – (Purchasing & Sales) – Purchasing)
**I have no idea how this works, do you? mind explain it to me ^.^