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1.标准偏差概念
标准偏差(Std Dev,Standard Deviation) -统计学名词。一种度量数据分布的分散程度之标准，用以衡量数据值偏离算术平均值的程度。标准偏差越小，这些值偏离平均值就越少，反之亦然。标准偏差的大小可通过标准偏差与平均值的倍率关系来衡量。
例如，A、B两组各有6位学生参加同一次语文测验，A组的分数为95、85、75、65、55、45，B组的分数为73、72、71、69、68、67。这两组的平均数都是70，但A组的标准差应该是17.078分,B组的标准差应该是2.160分，说明A组学生之间的差距要比B组学生之间的差距大得多。
标准偏差又分为总体标准偏差与样本标准偏差
，
代表所采用的样本X1,X2,…,Xn的均值。
，
代表总体X的均值。
= (200+50+100+200)/4 = 550/4 = 137.5
= [(200-137.5)^2+(50-137.5)^2+(100-137.5)^2+(200-137.5)^2]/(4-1)
stddev_pop(),stddev_samp(),stddev()
select col, stddev_pop(num),stddev_samp(num),stddev(num) as stddev_col from ( select 'A' as col, '1' as num union all select 'A' as col, '2' as num union all select 'A' as col, '3' as num union all select 'B' as col, '1' as num union all select 'B' as col, '2' as num ) as a group by col ;
查询结果：
select col, stddev_pop(num),stddev_samp(num),stddev(num) as stddev_col from ( select 'A' as col, '1' as num union all select 'A' as col, '2' as num union all select 'A' as col, '3' as num union all select 'B' as col, '1' as num union all select 'B' as col, '2' as num ) as a group by col
查询结果
由上可看出，hive中stddev()函数默认计算总体标准偏差，spark 中stddev()函数默认计算样本标准偏差
select col, stddev(num) over(partition by col) as stddev_col from ( select 'A' as col, '1' as num union all select 'A' as col, '2' as num union all select 'A' as col, '3' as num union all select 'B' as col, '1' as num union all select 'B' as col, '2' as num ) as a
查询结果：
select col, stddev_pop(num),stddev_samp(num),stddev(num) as stddev_col from ( select 'A' as col, '1' as num union all select 'B' as col, '2' as num ) as a group by col ;
查询结果：
（2）spark
select col, stddev_pop(num),stddev_samp(num),stddev(num) as stddev_col from ( select 'A' as col, '1' as num union all select 'B' as col, '2' as num ) as a group by col ;
查询结果：
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