分组聚合，就是先分组再排序，可以的话顺手标个排名；如果不想分组也可以排名；如果不想分组同时再去重排名也可以
Oracle和SQL server的关键字是over partition by
mysql的无关键字row_number() over (partition by col1 order by col2),表示根据col1分组，在分组内部根据col2排序 需求：给username加上唯一标示id
背景：需要一个纬度表，里面有仅仅username的唯一标示，因为hive中不存在自增id 但是有弊端，如果最大值有多个，那么就会出现多个最大值，so，要动态的ROW_NUMBER() OVER(
[PARTITION BY column_1, column_2,…]
[ORDER BY column_3,column_4,…]
)-- 建表
USE db_03;
DROP TABLE IF EXISTS employee;
create table employee (empid int ,deptid int ,salary decimal(10,2));
insert into employee values(1,10,5500.00);
insert into employee values(2,10,4500.00);
insert into employee values(3,20,1900.00);
insert into employee values(4,20,4800.00);
insert into employee values(5,40,6500.00);
insert into employee values(6,40,14500.00);
insert into employee values(7,40,44500.00);
insert into employee values(8,50,6500.00);
insert into employee values(9,50,7500.00);
SELECT * FROM employee;SELECT *, Row_Number() OVER (partition by deptid ORDER BY salary desc) rank FROM employee
select distinct
price,
row_number() over (order by price)
from
products
order by
price;price | row_number
---------+------------
300.00 | 1
300.00 | 2
400.00 | 3
500.00 | 4
600.00 | 5
600.00 | 6
700.00 | 7
800.00 | 8
800.00 | 9
900.00 | 10
1100.00 | 11with prices as (
select distinct
price
from
products
)
select price,row_numer()over(order by price) from prices;price | row_number
---------+------------
300.00 | 1
400.00 | 2
500.00 | 3
600.00 | 4
700.00 | 5
800.00 | 6
900.00 | 7
1100.00 | 8select deptid,salary
from employee a
where 2 > (
select count(1)
from employee b
where a.salary<b.salary and a.deptid=b.deptid
)
order by a.deptid,a.salary desc;SET @row=0;
SET @groupid='';
select a.deptid,a.salary
from
(
select deptid,salary,case when @groupid=deptid then @row:=@row+1 else @row:=1 end rownum,@groupid:=deptid from employee
order by deptid,salary desc
)a
where a.rownum<=2;SELECT t.stuid,
t.stuname,
t.score,
t.classid
FROM stugrade t
where t.score = (SELECT max(tmp.score) from stugrade tmp where tmp.classid=t.classid)
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