SQL：表列的缺失百分比和唯一计数答案

【问题标题】：SQL: Percentage Missing and Unique Counts of Table ColumnsSQL：表列的缺失百分比和唯一计数
【发布时间】：2021-04-29 19:28:54
【问题描述】：

假设我有一个有 4 列的表格。我想知道的每一列：

缺失值的百分比（空计数）和
唯一计数

如果我有一个包含 A B C 和 D 列的表，例如，上述情况的预期结果是：

Column_Name | PctMissing | UniqueCount
A           | 0.15       | 16
B           | 0          | 320
C           | 0.3        | 190
D           | 0.05       | 8

【问题讨论】：

用您正在使用的数据库标记您的问题。

标签： sql pivot unique missing-data calculation

【解决方案1】：

如果您知道列数，我可能只使用union all：

select 'a' as Column_Name,
  1.0*count(case when a is null then 1 end)/count(*) as PctMissing,
  count(distinct a) as UniqueCount
from t
union all
select 'b' as Column_Name,
  1.0*count(case when b is null then 1 end)/count(*) as PctMissing,
  count(distinct b) as UniqueCount
from t
union all
select 'c' as Column_Name,
  1.0*count(case when c is null then 1 end)/count(*) as PctMissing,
  count(distinct c) as UniqueCount
from t
union all
select 'd' as Column_Name,
  1.0*count(case when d is null then 1 end)/count(*) as PctMissing,
  count(distinct d) as UniqueCount
from t

Fiddle Demo

根据您的数据库，还有其他方法，但可能比union all 更令人困惑。

【讨论】：

【解决方案2】：

我会这样写：

select 'a' as column_name,
       avg(case when a is null then 1.0 else 0 end) as missing_ratio,
       count(distinct a) as unique_count
from t
union all
select 'b' as column_name,
       avg(case when b is null then 1.0 else 0 end) as missing_ratio,
       count(distinct b) as unique_count
from t
union all
select 'c' as column_name,
       avg(case when c is null then 1.0 else 0 end) as missing_ratio,
       count(distinct c) as unique_count
from t
union all
select 'd' as column_name,
       avg(case when d is null then 1.0 else 0 end) as missing_ratio,
       count(distinct d) as unique_count
from t;

【讨论】：