第四章 数据查询（上）

查询语句：

select <列名> 

from <表名>

[where <查询条件表达式>]

[ order by <排序的列名>[asc或desc] ]

　　asc升序（缺省）  

　　desc降序

使用as来命名列（取别名）

select Scode as 学员编号, Sname as 学员姓名, Saddress as 学员地址

from students

where Saddress<>‘河南新乡’

select FirstName + ‘.‘ + LastName AS ‘姓名‘ from Employees

查询空行

select SName from students where Semail IS NULL

查询非空行

select sname from students where not semail is null

或

select sname from students where semail is  not   null

限制固定行数

select top 5 sname, saddress 

from students where ssex = 0

返回百分之多少行

select top 20 percent sname, saddress 

from students where ssex = 0