数据库

索引与函数的关系：

1.存储过程

2，索引

3.触发器

4.多表查询

 

5.数据库函数

6.视图

5.数据库优化与效率

1，select  class,count(*) from s_table where Score> (select avg(Score) as Score from s_table )  group by class ;


1，select c_Name,count(Score) from s_table group by C_Name

2.select C_Name,sum(score)as 数量之和 from s_table group by C_Name
group by于聚合函数如sum，max，min，avg，first，last，count之间的关系。使用这些聚合函数时需要group by的配合。
3.group by与order by的区别

首先order by id desc是按id降序排列，asc是按升序排列。而group by对数据进行分组，将数据划分若干个区域。







delete from s_table where C_Name not in (select max(C_Name) from s_table group by C_Name)

select * from s_table where C_Name in (select C_Name from s_table group by C_Name having count(C_Name)>1)

delete from s_table where C_Name in (select C_Name from s_table group by C_Name having count(C_Name)>1)