SQL:表列的丢失和唯一计数百分比

假设我有一个4列的表。 我想知道的每个专栏的：

• 缺少值的百分比（空计数）和
• 唯一计数

如果我有一个列为a、B、C和D的表， 上述情况的预期结果将是，例如：

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

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如果您知道列的数量，我可能会使用

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

根据您的数据库，还有其他方法，但可能比union all更令人困惑

我将这样写：

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;

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用你正在使用的数据库标记你的问题。谢谢戈登！确实帮了我很多。。。