oracle的minus 返回第一个表中有、第二个表中没有的数据

CREATE TABLE union_tab_1 (
id INT,
val VARCHAR(10)
);

CREATE TABLE union_tab_2 (
id INT,
val VARCHAR(10)
);

INSERT INTO union_tab_1 VALUES(1, 'A');
INSERT INTO union_tab_1 VALUES(2, 'B');
INSERT INTO union_tab_1 VALUES(3, 'C');

INSERT INTO union_tab_2 VALUES(1, 'A');
INSERT INTO union_tab_2 VALUES(1, 'A');
INSERT INTO union_tab_2 VALUES(2, 'B');
INSERT INTO union_tab_2 VALUES(4, 'D');

SQL> SELECT * FROM union_tab_1
2 MINUS
3 SELECT * FROM union_tab_2;

ID VAL
---------- --------------------
3 C

SQL> SELECT * FROM union_tab_2
2 MINUS
3 SELECT * FROM union_tab_1;

ID VAL
---------- --------------------
4 D