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 

推荐：http://www.cnblogs.com/roucheng/p/3504463.html