新建两个表：
create table guotest1
(
sName varchar2(10),
Gender varchar2(10)
)
create table guotest2
(
sName varchar2(10),
Gender varchar2(10)
)
插入数据：
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返回的是两个查询结果表的差集
例如：select * from guotest1 where rownum <= 4 minus select * from guotest1 where rownum <= 2
返回结果：
"SNAME","GENDER"
"c","f"
"d","f"
2 minus返回的记录总是来自于左边的表
例如：select * from guotest1 minus select * from guotest2
返回结果：
"SNAME","GENDER"
"a","m"
"b","m"
"c","f"
"d","f"
像是说：如果左边的表中的某个记录存在于右边的表中，则该记录不会返回