MYSQL order 、group 与row_number详解

一、order by

order by A ASC, B DESC,C ASC …

上述语句会先按照A排序，当A相同的时候再按照B排序，当B相同的再按照C排序，并会不按照ABC组合一起排序

二、group by

group by A,B,C…

select 中的字段必须是group by中的字段，但是聚合函数中的参数字段没有必须出现在 group by中

select A , B ,count(*) from t1 group by A,B,C
select A , B ,C,count(*) from t1 group by A,B,C
select A , B ,count(C) from t1 group by A,B,C
# 语法错误，应该使用having
select A , B ,count(C) as countABC  from t1  where countABC >3  group by A,B,C
select A , B ,count(C) as countABC  from t1  having countABC >3  group by A,B,C select A , B ,count(C) from t1 group by A,B,Cselect A ,count(*) from t1 group by A 与 select A ,count(A) from t1 group by A 
与 
select A ,count(B) from t1 group by A  效果一致select A ,count(distinct B) from t1 group by A 
与
select A ,B,count(*) from t1 group by A ,B 效果一致select A ,sum(B) from t1 group by A 
select A ,max(B) from t1 group by A 
select A ,min(B) from t1 group by A 

三、row_number 窗口排名函数

row_number() over ( [partition by column] order by order_expression [asc|desc]) as ranking
该方法常用于排名、连续登陆数统计，部门业绩排名

row_number 无并列排名，强制连续序号 例如 1，2，3
rank 允许并列排名，后续序号不连续 例如 1，1，3
dense_rank() 允许并列排名，后续序号连续 例如 1，1，2

count(*) 统计所有行数，包括NULL
count(column) 统计column字段非NULL的行数