Subquery grouping changes the result

Question

Accepted Answer

Wrapping the UNION in a subquery forces it to run first, producing {1, 2, 3}. Then INTERSECT with c ({2, 4}) keeps only the common value: {2}. This is the result most people intuitively expect from `a UNION b INTERSECT c` - but without the parentheses the answer is different because INTERSECT binds more tightly than UNION.