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