新建兩個表：
create table guotest1
(
sName varchar2(10),
Gender varchar2(10)
)
create table guotest2
(
sName varchar2(10),
Gender varchar2(10)
)
插入數據：
insert into guotest1 values('a', 'm');
insert into guotest1 values('b', 'm');
insert into guotest1 values('c', 'f');
insert into guotest1 values('d', 'f');
insert into guotest2 values('e', 'm');
insert into guotest2 values('f', 'm');
insert into guotest2 values('g', 'f');
insert into guotest2 values('h', 'f');
1 minus返回的是兩個查詢結果表的差集
例如：select * from guotest1 where rownum <= 4 minus select * from guotest1 where rownum <= 2
返回結果：
"SNAME","GENDER"
"c","f"
"d","f"
2 minus返回的記錄總是來自於左邊的表
例如：select * from guotest1 minus select * from guotest2
返回結果：
"SNAME","GENDER"
"a","m"
"b","m"
"c","f"
"d","f"
像是說：如果左邊的表中的某個記錄存在於右邊的表中，則該記錄不會返回
