SQL SERVER中PERCENTILE_CONT和PERCENTILE_DISC

PERCENTILE_CONT和PERCENTILE_DISC

看下面一组SQL语句：

WITH test
as
(
    select N‘LeeWhoeeUniversity‘ as name,10 as score
    UNION ALL
    select N‘LeeWhoeeUniversity‘,20
    UNION ALL
    select N‘LeeWhoeeUniversity‘,30
    UNION ALL
    select N‘LeeWhoeeUniversity‘,40
    UNION ALL
    select N‘LeeWhoeeUniversity‘,50
    UNION ALL
    select N‘DePaul‘,60
    UNION ALL
    select N‘DePaul‘,70
    UNION ALL
    select N‘DePaul‘,80
    UNION ALL
    select N‘DePaul‘,90
    UNION ALL
    select N‘DePaul‘,100
)
select name,score
,PERCENT_RANK() over(partition by name order by score) as per_rnk
,PERCENTILE_CONT(0.5) WITHIN GROUP (ORDER BY score) over(partition by name) as percont0_5
,PERCENTILE_CONT(0.6) WITHIN GROUP (ORDER BY score) over(partition by name) as percont0_6
,PERCENTILE_CONT(0.7) WITHIN GROUP (ORDER BY score) over(partition by name) as percont0_7
,PERCENTILE_CONT(0.75) WITHIN GROUP (ORDER BY score) over(partition by name) as percont0_75
,PERCENTILE_DISC(0.5) WITHIN GROUP (ORDER BY score) over(partition by name) as perdist0_5
,PERCENTILE_DISC(0.6) WITHIN GROUP (ORDER BY score) over(partition by name) as perdist0_6
,PERCENTILE_DISC(0.7) WITHIN GROUP (ORDER BY score) over(partition by name) as perdist0_7
,PERCENTILE_DISC(0.75) WITHIN GROUP (ORDER BY score) over(partition by name) as perdist0_75
from test

运行结果：

name                          score    per_rnk    percont0_5    percont0_6    percont0_7    percont0_75    perdist0_5    perdist0_6    perdist0_7    perdist0_75
LeeWhoeeUniversity    10           0                 30                       34                     38                        40                     30                 30                  40                       40
LeeWhoeeUniversity    20           0.25           30                       34                      38                       40                      30                30                   40                       40
LeeWhoeeUniversity    30           0.5              30                       34                      38                       40                      30                30                   40                       40
LeeWhoeeUniversity    40           0.75            30                       34                      38                      40                        30               30                   40                       40
LeeWhoeeUniversity    50           1                 30                       34                      38                       40                       30                30                   40                       40
DePaul                    60            0                80                       84                      88                       90                       80                80                   90                       90
DePaul                    70            0.25          80                        84                      88                      90                       80                 80                  90                       90
DePaul                    80            0.5             80                       84                      88                      90                       80                 80                  90                        90
DePaul                    90            0.75          80                        84                      88                      90                       80                 80                  90                       90
DePaul                  100           1                80                        84                       88                     90                        80                80                  90                        90

简单理解，PERCENT_RANK前面介绍过，求出score百分比后，PERCENTILE_CONT和PERCENTILE_DISC就是根据百分比求出对应的score。但不同的是，某一百分比没有对应的score时，PERCENTILE_CONT会根据百分比的偏差计算出一个新值，此值可能并不存在于score中。PERCENTILE_DISC得出的是偏向于最近百分比对应的score值，因此此值肯定存在于score中。

如PERCENTILE_CONT(0.6)对应的值为34，偏差值我猜测是这样计算出的：百分比75%和50%PERCENTILE_CONT计算出的值分别是40和30.那么等式：(40-30)/(0.75-0.5)=新偏差值/0.6-0.5。PERCENTILE_CONT(0.6)在50%上的偏差值=4。所以，PERCENTILE_CONT(0.6)对应PERCENTILE_CONT(0.5)+4=34。

再看PERCENTILE_DISC(0.6)，直接将最近的0.5计算出的，PERCENTILE_DISC值取过来了。

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