oracle的minus返回第一个表中有、第二个表中没有的数据

CREATE TABLE hovertree_union_1 (  
id INT,  
val VARCHAR(10)  
);  
  
CREATE TABLE hovertree_union_2 (  
id INT,  
val VARCHAR(10)  
);  
  
INSERT INTO hovertree_union_1 VALUES(1, 'A');  
INSERT INTO hovertree_union_1 VALUES(2, 'B');  
INSERT INTO hovertree_union_1 VALUES(3, 'C');  
  
INSERT INTO hovertree_union_2 VALUES(1, 'A');  
INSERT INTO hovertree_union_2 VALUES(1, 'A');  
INSERT INTO hovertree_union_2 VALUES(2, 'B');  
INSERT INTO hovertree_union_2 VALUES(4, 'D');  
  
SQL> SELECT * FROM hovertree_union_1  
2 MINUS  
3 SELECT * FROM hovertree_union_2;  
  
ID VAL  
---------- --------------------  
3 C  
  
SQL> SELECT * FROM hovertree_union_2  
2 MINUS  
3 SELECT * FROM hovertree_union_1;  
  
ID VAL  
---------- --------------------  
4 D 

-- 何问起 hovertree.com