1.去除重复数据,针对已查询出的整个结果集进行数据重复处理,而不是针对某一列。
select distinct sName from student
select distinct sName,sAge from student
2.TOP关键字获取前几条数据,一般都与order by连用
select top 5 * from TblScore order by tMath asc --取数学成绩最低的前5个,升序排列
select top (2*2) from TblScore order by tMath desc--取数学成绩最高的前5个,降序排列,top后面如果是表达式,必须括起来
3. select top 30 percent * from TblScore order by tMath asc --取数学成绩最低的前%30,升序排列,百分比如果不是整数条,会向上取整(夺取一条)