MySQL 8.0 引入的窗口函数，本节主要介绍row_number,cume_dist,percent_rank

row_number,cume_dist,percent_rank

1. row_number()：显示行号

2. cume_dist：计算某个值在一组有序的数据中累计的分布

    计算结果为相对位置/总行数,返回值为(0,1]
   注意：对于重复值，计算的时候，取重复值的最后一行的位置

3. percent_rank：和cume_dist 的不同点在于计算分布结果的方法
   计算方法为（相对位置-1）/（总行数-1）

   注意：对于重复值，计算的时候，取重复值的第一行的位
   

root@my3308.sock-8.0.11>[test]>select ROW_NUMBER() OVER w AS 'row_number', 
  c1, 
  CUME_DIST()  OVER w AS 'cume_dist',PERCENT_RANK() OVER w AS 'percent_rank'  
  from x window w as (order by c1);
+------------+------+-----------+---------------------+
| row_number | c1   | cume_dist | percent_rank        |
+------------+------+-----------+---------------------+
|          1 |    1 |     0.125 |                   0 |
|          2 |    7 |     0.375 | 0.14285714285714285 |
|          3 |    7 |     0.375 | 0.14285714285714285 |
|          4 |   10 |     0.625 | 0.42857142857142855 |
|          5 |   10 |     0.625 | 0.42857142857142855 |
|          6 |   11 |      0.75 |  0.7142857142857143 |
|          7 |   24 |         1 |  0.8571428571428571 |
|          8 |   24 |         1 |  0.8571428571428571 |
+------------+------+-----------+---------------------+
8 rows in set (0.00 sec)

#cume_dist
root@my3308.sock-8.0.11>[test]>select 1/8 as 'cume_dist_c1=1';
+----------------+
| cume_dist_c1-1 |
+----------------+
|         0.1250 |
+----------------+
1 row in set (0.00 sec)

root@my3308.sock-8.0.11>[test]>select 5/8 as 'cume_dist_c1=10';
+-----------------+
| cume_dist_c1-10 |
+-----------------+
|          0.6250 |
+-----------------+
1 row in set (0.00 sec)

#percent_rank
root@my3308.sock-8.0.11>[test]>select (2-1)/(8-1) as 'percent_rank-c1=7';
+-------------------+
| percent_rank-c1-7 |
+-------------------+
|            0.1429 |
+-------------------+
1 row in set (0.00 sec)

root@my3308.sock-8.0.11>[test]>select (7-1)/(8-1) as 'percent_rank-c1=24';
+--------------------+
| percent_rank-c1=24 |
+--------------------+
|             0.8571 |
+--------------------+
1 row in set (0.00 sec)