限制结果集的行数
最大的作用，用于分页，防止信息过多，过载导致机器当机。
限制结果集
select * from T_Employee
order by FSalary DESC
选出前三名：
select top 3 * from T_Employee
order by FSalary DESC
检索按照工资从高到低排序
从第六位开始一共三个人的信息：
select top 3* from T_Employee
where FNumber
not in
(select top 5 FNumber from T_Employee order by FSalary DESC )
order by FSalary DESC
