Precedence ordering

UNION and EXCEPT/MINUS have the same precedence order. If these operators appear in the same query expression, SQL executes them from left to right. INTERSECT, however takes higher precedence than the other set operations. Thus, if you use INTERSECT with other set operators, SQL executes the INTERSECT operation first.

In the following examples, S1, S2, S3, and S4 represent union-compatible SELECT statements.

S1 UNION S2 EXCEPT S3 UNION S4 => (((S1 UNION S2) EXCEPT S3) UNION S4)
S1 UNION S2 INTERSECT S3 MINUS S4 => ((S1 UNION (S2 INTERSECT S3)) 
EXCEPT S4)

To avoid confusion or to force a certain execution order, use parentheses:

(S1 UNION S2) INTERSECT (S3 MINUS S4)